CRTs are way less finicky with weird sync signals than contemporary flat panel TVs, so chances are whatever device you’re using (would be helpful to know what devices you’re using) is outputting an off-spec sync or refresh rate that your display connected to your OSSC doesn’t like. Some issues like this can be mitigated by adjusting the OSSC’s settings.
If it’s really the splitter that’s at fault (I assume you’ve tried other combinations of ports and cables, and powering the splitter if it can be powered), and you really only need two outputs, and it needs to be SCART, I suggest a gscartsw, which has 8 SCART inputs and 2 SCART outputs and is highly regarded in the retro gaming community.
A possible alternative would be to use an Extron CrossPoint as a distribution amp/splitter. I’ve done this with success, simultaneously duplicating each of composite video, S-Video, and YPbPr to two displays; but, since you’re using RGBS, you will need to attenuate the outgoing sync signal with one of these, and you will need SCART->BNC and BNC->SCART adapters if your devices can’t accommodate BNC, RCA, or DE-15/VGA (Which are all cheaper adapters than SCART to/from anything).