You can get away with one bulb and one filter and one switch if you put them all in series before a switch over valve. That way the bulb charges either motor and there is no chance of either motor affecting the other. Tank to filter to bulb (or bulb to filter) to switch...two lines out of switch, one to each motor. You will not be able to run both motors at the same time, however. It really is more a factor of your particular layout, but there are many options possible.

Nothing wrong with Halibut Steve's layout, either, if you're just talking about two outboards. Each check bulb will stop any feedback from one motor to the other if the bulbs are hung vertically. Any sucking action in the dead line will only seat the check ball more firmly. I wouldn't use this method with a inboard/outboard and outboard kicker combination though. The fuel in a carbureted i/o could slosh the float around and unseat the fuel check valve allowing small amounts of fuel to feed back into the kicker line. No problem at first, but after it does this a number of times then the valve becomes totally unseated due to the lack of fuel in the bowl and the kicker will suck air through the i/o's carb.