They collect separately. Even after the truce, they are separate countries and collect separately.
Peacetime bonus Income
We know that the the income goes up immediately on the chart. But, the allies were thinking that they got that money at the beginning of their turn. I say that it goes up immediately so they can declare war. But, they don’t actually receive the money until their collect income phase on there next turn. Who’s right?
They can declare war immediately once they reach a certain income. They don’t have to collect that income, so they do not have to wait until their turn to declare war.
Yes Munck is correct. It’s not about how much money you collect, it’s about your level of income. Considering that you can declare war at any point in time during the game, you might want to declare immediately before your income has a chance of going down due to a country you are already at war with lowering your income before you declare. That is unlikely though.
When Munck and I were playtesting the game at HBG I wasn’t able to go to the bathroom because I didn’t want to declare war against Japan with the Commonwealth unless they tried to enter the Mediterranean. The Japanese player did indeed try to enter, I declared war, then I was off to the bathroom. The point is that you have to give some thought as to when you make the declaration, unlike A&A when you only declare when you begin your own combat move.
While GHG and Munck are correct about income level and declaring war, I don’t think they answered your question.
What you said is correct. Your income on the chart goes up immediately (so you can declare war), but you don’t actually collect the income until the “Place Units and Collect Income” phase of your turn.