I moved to Austria last year and got a new license here recently.
Since I intend to use my Croatian call in Croatian SOTA activations and the Austrian in Austrian, is there a way to use two Callsigns simultaneously in logging or do I have to change it to the Austrian one?
You enter the call sign you used on the air when you enter the activation. So you can use one or the other call sign. They all get saved in your account where your home association and call sign is listed. As you have moved to Austria, I suggest you log in and change your call sign to your Austrian call and your home association to Austria. That way your account reflects your situation but all your SOTA effort so far is still available to you.
My FT-817 has an ALC meter that is used particularly when operating datamodes. It accurately reflects how many beers I will drink after an activation…