/u/SeanVo is right - splitters cost a lot of power. If you can, you could put an LNA between the antenna and the input to the splitter. If your dongles have bias voltage available, you may be able to power it from one of them if you use a splitter that has a DC path between the common input and at least one of the outputs. If it were me, however, I think I'd just power the LNA directly.

PS - If you're somewhat electronically-inclined you could build an active splitter with these little jewels.