Same affect as feeding two antennas from a single amp. Although it is
sometimes a good way to get more watts/$

Although you have stipulated same feedline lengths there could be some
phase delays inside the amps you dont know about (eg filter components).
You should of course be able to compensate for this by varying line
length. A difference in output power from each will also skew the
radiation pattern.

If the antennas are thus fed in phase and the amps produce the same
power you will get max radiation perpendicular to a line drawn between
the antennas and minimum radiation in line with them. You can play with
all kinds of phase delay/line length and spacing variations to get a
good mix of patterns. They interfere from the standpoint that you get
phase addition and cancellation of the two signals combining. This is
how driven array antennas work.

Do you have a specific application in mind?

Cheers Bob VK2YQA

Ponder This wrote:
Will the output of the two amplifiers add together in directions where
the signal is in phase, or will it act like two different signals and
interfere with each other?

-Curious