Signal strength
A few things to consider.
1. The "bars" are only a relative indication of signal strength. The same signal on the same antenna will not necessarily show the same number of bars - this isn't a caliberated measurement.
2.A two way splitter has three connections. Check to see that your antenna is connected to the "input", and that each receiver is connected to "output".
3.To rule out the possibility of a bad splitter, bypass the splitter and check signal levels using a direct connection. This would also give you some idea of just how much signal loss the splitter introduces.
4. Of course, it's also possible thatthe use of an external antenna and the presence of a strong signal is desensing the receivers front end, thus resulting in what appears to be poorer sensitivity.
Hope this helps to figure out what's going on.