The only time I'd ever use one LPDA and one
whip would be if the second LPDA got broken or fell out of the truck.
Diversity was designed to work best when the signal levels are similar.
In many ways, it would be like having 2 microphones on a
speaker - one a
condenser mic (the LPDA) and one a SingStar Karaoke mic (the
whip) and trying to mix the two - all the while while the
presenter is moving about and dropping in and out of the lobes of the
condenser's coverage
pattern.
There is a company (and right now I can't remember their name) who are making a combination LPDA and dipole in the same assembly. This gives you polarisation diversity which in many cases is as good and in some cases better than the spatial diversity normally used with LPDAs. It comes with 2 connectors on the bottom for 2 coaxes to connect.
One however MUST remember that LPDAs are a tool, and one that can be used wrong just as easily as they can be used right.
There are some situations where they are absolutely the wrong tool to use and
stock whips will give much better performance.
Oh and you would only use at most ONE LPDA per
system if it's the transmit antenna for an
IEM rig... (Noting that multiple antennas, appropriately spaced, can easily outperform using combiners to sum multiple IEMs together. Again an area where you need to understand ach tool's characteristics and how to use it properly...