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Yes, it's possible.
How? Well (laugh) you take the special stack cable and connect one end to one of the 3750X stack's ports and the other to one the 3750G stack's ports. Ideally, repeat (to have dual stack ring connections).
Of course, there's other requirements such as the StackWise software is required to be the same major version (but matching the minor version is a really good idea - matching IOS versions is the easiest way to accomplish this). Also, the -12S has SDM templates unique to that model, so you want to insure it's configured not to use those. The 3750X's LANBASE license also won't stack with the 3750G, you need IPBase or IPServices. Then you need to decide stack member numbering as each device's ports, in the config, will reflect that.
Also be aware, stacking StackWise and StackWisePlus 3750s results in the latter needing to give up some of its improvements.
More information can be found in the IOS reference and guide manuals for your IOS version.
