Yes you can use one GPS antenna to feed info into more than one reciever.
The issues are they are not seperate systems hence validating one against another and redundancy.
I 'm not sure from your post if you have got plain antennas or reciever/antenna combinations.
Assuming that they are plain antennas using RF cable to hook up to the reciever, you can use a splitter to do this.
Another limitation is that GPS antennas are usually powered so it needs a dc block in it to avoid overloading the antenna.
Be warned about cheap splitters, as you are halving the signal between recievers you need the smallest losses or use an amplifier splitter. I might have a spare splitter but in middle of house move so finding it could be a problem.
If you are using a combination reciever/antenna then the signal will be RS 232 or a propriatry string. If you losses are not too great (cheep cable, corroded connections etc) then you can probbably get away with parallelling them up or inverst in an rs232 distribution box - not too expensive.