Well it wouldn't be anything that I would recommend, and if your MOBO is Dual Channel with 4xDIMM slots (1) stick of DDR3-1333 + (1) stick of DDR3-1600 would lead to problems either immediately of down the line with memory intensive apps.
Clearly, at best you'll need to look at CPU-z 'SPD' tab and look for an overlapping Frequency ~667 and CAS Timings and then use the worst of each; example:
DDR3-1333 (667MHz) 9-9-9-24 @ 1.50v
DDR3-1333 (667MHz) 8-8-8-24 @ 1.50v
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DDR3-1333 (667MHz) 9-9-9-24 @ 1.50v ; set the DRAM Frequency DDR3-1333 and CAS Timings.
Next, some MOBO's must have the Channels be the same size, so in the example 2GB Channel 0 and 4GB Channel 1 the end result might be 4GB total and you'll loose 2GB of the 4GB stick and so in that case simply use the (1) 4GB stick.
RAM is dirt cheap, buy a matched pair.
Q - What MOBO?
Q - What CPU?
Q - What exact stick/sets? ; provide links.
Q - What OS?