I am trying to design a bot, it has to measure the temp I am using an 8051 so the adc is taking up a lot of i/o pins so I am thinking of switching it out with a thermostat or put another controller and interface them (which also has the advantage of letting me add more features) But I wanted to make sure in general is it a good design to add multiple micro-controllers on the same pcb, will it cause hardware problems?
Will it cause hardware problems? Theoretically, no. This is done quite often, so there is no inherent limitation there. If you design the board badly on the other hand, well that's just bad design!
Using multiple microcontrollers is a great thing, because it lets you parallelize your tasks and can make life a lot easier on that front. However, with that comes a somewhat more complex firmware as you now need to synchronize the microcontrollers, make them communicate, and handle relevant errors.
In short, in general it's not at all a bad idea to put multiple controllers on the same PCB. Just be sure you understand the implications on your firmware design. Sometimes it may be a lot simpler to use a slightly larger microcontroller.