i asked it before and found the answer my self after a while. (i have 5 renforts in the same account now)
when you activate the first renforts for the next 24 hours, you cant activate another one untill the 24 hours reset. if you even click on the renforts for the second renforts, you will get an error massage that you can only activate only one renforts every day.
however , i was once able to activate 2 at the same day by ranking the first used renforts up, and then using a second renforts as a new one. this is of course a special case , but it is very important if you have diffrent renforts at diffrent lvls. and the ones with high lvl have allready won a new rank , but kept beeing used as a renforts to help the team mates...