rfmw, thank you for your answer.
I was trying to learn something about transmission lines, and do as you suggested, but I found out that this subject is quite complex, and it would take me at least one or two weeks to learn enough to make this work that way. It is a lot of time for just one little think. I also found one way to do this (attachment), but I don't know, if it works as it should. I basically made a rectangle under both antennas, and used Driven Terminal mode. Than I assigned wave port, and chose one antenna as conducting object, and one as reference conductor. In this way both antennas radiate, but I don't know if this is right... also the radiation patten changes drastically when changing the size of the wave port.... is this a solution?