I don't know much about them during WW1, just that they were in the army. Are there documents available for viewing of recruited soldiers in the Austro Hungarian army during WW1, such as recruitment papers? They were both Croats, if that helps in any way.
I'm sure you know it, but what ethnicity your relatives are, what post-Versailles country they remained in after the war?
The Austro-Hungarian army was, at the regimental level at least, ethnically homogeneous within units especially in the early years, and it could help people narrow down where to look, given the number of successor states and the sometimes uneven and scattered nature of the records upon the breakup of the monarchy. That of course won't really help when (in an extreme example) for instance an ethnic Slovak regiment's records might be in Slovakia, the Czech Republic, or Austria, depending upon how diligent people were in transferring documents as necessary with each time the nations split apart.
If they are from the area around Split and Solin they would have been recruited into Infantry Regiment NR 22 of 16 corps. After the rationalisation of the Common army in June 1918, they could have been transferred to a newly raised regiment NR 122. I'd [email] (http://www.oesta.gv.at/site/6156/default.aspx) them and ask if they carry regimental files from IR NR 22, from 1914-1918. They will be able to assist. The team are fantastic, and are always willing to help.