They can be BUT, they will be at different temps. The thermostat will sense whatever the probe is placed on and adjust output accordingly.
For example (and making up numbers) you have 3 runs of 3" 6 watt heat tape and another run of 11" 20 watt. If the probe is on the 6 watt tape and maintaining 80 degrees, the 20 watt will be receiveng several times as much power as it needs to maintain the same temp.
You can use that to your advantage but will require some extra work and IMO is somewhat dangerous to your herps.
One scenario is using close wattages like some 4 watt and some 6 watt. The higher wattage could be on cages that need more heat input to maintian the desired heat while the lower wattage is on tubs that require less. The danger is not all the heat tape will be the same temp. One way you could "fix" the difference is having a dimmer on the higher wattage tapes so they can be adjusted down to desired temp range.
ALL of that is complicated and requires knowledge of electricity and wiring to set up in the first place and a high level of dilligence to ensure the individual runs are at the right temps.
IMO, you are MUCH better off with multiple thermostats!!!
My collection of thermostats is now at.....
4 - herpstat Pro 4 channel units (16 total zones)
13 - Helix DBS1000
22 - Habistat (older thermosatats, came on used cages I bought)
2 - Ranco (not currently in use but I still have them
Staggering huh?? That's a total of FIFTY THREE seperate controllable temperature zones!!!
The real overkill is the habistats, 16 are being used at two per cage, one controlling floor heat and the other controlling an overhead CHE. (Ceramic Heat Emitter)
-----
Thanks,
Dave Colling
www.rainbows-r-us-reptiles.com
0.1 Wife (WC and still very fiesty)
0.2 kids (CBB, a big part of our selective breeding program)
LOL, to many snakes to list, last count:
26.49 BRB
20.21 BCI
And those are only the breeders 
lots.lots.lots feeder mice and rats 
