No sorry. I dont like a system with separate attacks. I dont like that the US stack attack separately first, then followed by a separate UK attack, because this force the Axis stack to doubble-defend. The defender rolls twice as many dice as they should and that is unbalanced.
Another issue is the management of resolving like 20 small battles in a global game. With joint attacks you resolve all this battles one time each. With separate attacks, then you’ll have to place all this units on the Battleboard twice. It will be like resolving 40 battles when you could have resolved 20, and that is time consuming. Joint battles will speed up the pace.
If you feel that joint attacks is too powerful, just make some rules. USA/UK are just friendly to USSR, but they are not allowed to share territory or attack together. Same with Japan and the Euro Axis. If the US/UK joint D-day is still too strong, just let Germany build the Atlantic Wall. Blockhouse cost 2, defend at 3 or less, can stack but not move.