Since you have the 2 sets already it can't hurt to try them together (chances are they'll be fine since both are 1600 CAS9 @1.5V) - If it were me I'd use them and just keep on mind that if stability issues arise that I might need to make some adjustments to speed or timings to find the balance where they both work properly together - doesn't hurt to try and usually there are not any problems with such closely spec'd modules and they may actually be the same modules with a different heatsink applied anyway.
I'm actually running 2x1GB OCZ platinum
rated at 1333 7-7-7-20 1.8V alongside a 2x1GB Crucial Ballistix
DDR3 1800 8-8-8-24 @1.9 -- Since the Crucial were no longer available (got them for a really good price but crucial discontinued them ) and OCZ had a good sale when I was buying a second set ---- Am running them both together at DDR3 1333 7-7-7-21 -1 @ 1.8V because this was the setting they both ran well at with no stability issues and the fastest throughput (I've also run them at DDR3 1600 8-8-8-24 for awhile but the system performance is better at 1333 7-7-7-21 than it was at 1600 since the memory controller on my PII x3 720 prefers 1333Mhz.)
So you should not have problems but there are times where you will but that usually just means a little tweaking is needed to get them working together correctly (just have to find the fastest timings they will both run stable at which can take some playing with at times!)