I did a quick search and couldn't find the answer that I was looking for.

I was playing a game today and my opponent was running a CR90A with TRCs. On one attack he spent two evades to change both of his dice to double hits.

My question is can you change both dice during one attack?

My initial thoughts were that you can only manipulate one dice during each attack, if you want to spend both evades. Thoughts?

Any card that says "while X, you (can) do Y" indicates both the timing window of the effect as well as the fact that it can be used only ONCE during that timing window. Check out the rules reference guide for backup on that.

So you are correct, he can only spend one evade token per attack.

Only one per attack.

You can use it once on each attack if you have two targets

You can use it once on each attack if you have two targets

You don't need two targets, just two attacks. You could make a front and side arc attack against the same (single) target for example.