Bubs, on and off works, but some times the system has a drama working out location and time.
You can use the card off on sequence, and this will generate a new file.
PC and I shared one unit all last year. Only drawback is your distance is halved, the average 1 hour speed usually gets recorded as 0 due to one not running a GPS when you out on the water.
And as Hardie indicated, you can claim the fastest session


The other way you can do it is to "split" the session in RSpeed, but you will need to record what time you change over the GPS.
What we did in the end, was the person who is able to spend most time dry (gybes and such) kept the NAVi on, and the other tried for 2 sec, 5 x 10 sec and Alpha when he has it
Hope this helps.
Mineral