I need to plug my balanced xlr (left and right) into my mixer which has unbalanced inputs, should I just connect two balanced xlr cables directly from the output into two unbalanced xlr inputs in the mixer?
You can just plug XLR to XLR in pretty much all situations but you should be careful about levels. XLR outputs on powered devices are usually line level, while XLR inputs are often (especially on a mixer) mic level or switchable between mic and line level. You should switch the inputs to line level, if possible, and start with the output level as low as possible and bring it up slowly and check your gain stages.
If the inputs are actually unbalanced (as I commented, this is rare - I've never seen an unbalanced XLR in before), then it depends on how it's wired. If pin 3 is grounded and pin 2 is connected to the input stage, you'll be fine. If pin 2 is grounded and pin 3 is connected to the input stage, you'll be fine but the signal will have reversed polarity. If somehow pins 2 and 3 are both connected to the input stage, you'll get no signal - this would be a very strange configuration that to me would indicate equipment I would want to get rid of ASAP.
If you XLR outputs are unbalanced (just as rare as XLR ins being unbalanced), or if you have any unbalanced signal going into a balanced XLR input, it can be done with simple adapters, but the best way to do it is to use a DI box of some sort.
Answering your final question (in comments) then yes, that's what they are there for. Balanced XLRs ought to be wired to industry standard, and fit both the output from sub-mixer and input on main mixer, but use the gain controls to achieve a sensible balance between the two.
A mixer will typically output Line level, whether balanced or unbalanced, on jacks, XLRs, or phonos. The XLR channel inputs however will doubtless expect mic level, which is much lower. (Balanced XLR Line Ins certainly exist, but only on professional level equipment, designed for use in electrically noisy environments such as TV studios.) If the first mixer also offers jack outputs, use them. Either into the Line In option on a couple of mixer channels or, if available maybe into a pair of Aux In or FX Return sockets.
I say again, tell us EXACTLY what two pieces of gear you're trying to connect, we can give a better answer.