Ivor,
Splitters are technically possible, but bearing in mind the frequency and relatively low power we are using, would probably cost more than all the rest of the setup. Also bear in mind that we are limited by the constraints of the licence-free frequency band we are using to a maximum of 500mW on a 1% duty cycle, so must ensure we do NOT generate any additional antenna gain in the system.
Best advice is to stick to the recommended antennas and maximise their effective position.
Regards
Peter