Was there a rational reason for Napoleon to attack Russia while the Peninsular War was still in progress?

Actually, I would argue Napoleon's plan to fight on two fronts (in Spain and in Russia) was quite rational.

1. In 1812 there was a real threat that Russia will attack Poland; in fact, Russian tzar was planning exactly this. Russia's military plan for a war agains Napoleon was prepared in 1810 by general Karl Ludwig von Phull in 1812. The plan starts with the assertion that Napoleon will be ready to fight against Russia only after successfully concluding the Spanish campaign. (So, at least you got general von Phull on your side of the argument. But there is a reason that Napoleon's name is much more well-known than von Phull's: Napoleon did not always follow the conventional wisdom when fighting his wars.) To which extent the plan was really used by the Russian military is not exactly clear (per Wikipedia's page):

It is disputed how involved Phull was in the Russian decision to engage in a scorched earth policy against Napoleon Bonaparte during his invasion of Russia.

However, von Phull in his plan did stress the logistical vulnerability of Napoleon's army (in Russia).

In Spring of 1812 Napoleon actually had two plans for Russian campaign, depending on who will attack first: Russia or France. Per Wikipedia's page:

The official political aim of the campaign was to liberate Poland from the threat of Russia. Napoleon named the campaign the Second Polish War to gain favor with the Poles and to provide a political pretext for his actions.

2. The real aim of the war was, of course, to maintain the Continental blockade against England. (In 1807 tzar Alexander I agreed to maintain the blockade, but quit in 1810.)

3. The French expectation was that the war will be primarily won near the border and will be over by September of 1812. If this were the case, the Peninsular War would have been irrelevant (as far as the invasion of Russia is concerned),

4. The alternative that you suggest would have been for Napoleon to finish the Spanish war first. The trouble is that France was fighting this war since 1807 and still was not done in 1812 after five years of fighting. Sometimes it takes decades to win a war against a guerrilla army (and, of course, you know many examples of this...). It is not even clear what does it mean to win such a war since the insurgents can simply bury their weapons and wait for the bulk of French troops to withdraw, and resume the war afterwards. (And Wellington's army would simply board their ships and go home for the time been, only to land back half a year later.)

At the same time, the Continental blockade would be failing.

5. In retrospect: Let's ignore the Peninsular war for a moment, suppose French could have used the bulk of its forces in the Iberian peninsula (which, by some estimates, numbered up to 300K). Say, 200K of these forces were to join the Grande Armée. OK, Napoleon could have used them to strengthen his push against St Petersburg in 1812 (Laurent Gouvion Saint-Cyr's troops) and even took it. Now what? (The administrative capital of Russia would temporarily move East, say, to Kazan on Volga, and Alexander I still would refuse to sign the peace treaty.) Having extra troops only worsens the logistical problems that Napoleon was facing in Russia (more troops to feed). And, ultimately, it was the logistics, not the lack of troops, which doomed Napoleon's army in Russia.

I will add further citations and references later on.

Here is an article by Harold T. Parker which is appropriately titled "Why did Napoleon Invade Russia? A Study in Motivation and the Interrelations of Personality and Social Structure". It opens by pointing out that Napoleon's closest advisers strongly advised him against invading Russia. Therefore, according to Parker, most of the explanation for why he did it anyway rests on Napoleon's own psychology and experience in a social context of rising nationalism.

His underlying disposition toward combative mastery and control (he really enjoyed a fight, especially when successful) was laid down in a Corsican family and town; his abilities and his dedication to hard work were strengthened by a French education, in the college of Brienne, in a French army then undergoing progressive reform, and in his reading of the philosophes and history; his Corsican style of craftiness was practiced in the factional island quarrels. The French Revolution opened up opportunities to his great military and administrative gifts and revealed his superiority to himself and to a large public; a youth who in his historical reading had resonated to Robert Bruce, the liberator of a small country, now resonated to Alexander, Caesar, and Charlemagne, heroes who had conquered space and founded institutions.

His personality and dexterous, crafty practices, formed outside the system of international diplomacy he was entering, approximated sufficiently its values and methods that he could operate within it. The French revolutionary army, which he transformed into an imperial professional force, the excellent French foreign service which he improved, and the European practice of arranging supporting alliances offered him instruments for the achievement of great successes on the battlefield and at the negotiating table. Great rewards reinforced the disposition to seek mastery of everything in view and to push events to the utmost. But great success and the habitual exercise of command from the self-isolation of his office blinded him to the reality of what was possible. Restless, illimitable striving eventually entrapped him into a double war against two relatively invulnerable powers, England and Russia. Like Louis XIV and Louis XV before him, he lost on both fronts.