The X3600 is a 9 channel receiver with 11 channel processing.You can add 2 external channels of amplifucation to it to give you a 7.1.4 setup or do 7.1.2 out of the box. The X3600 is a 7 channels AV receiver than can only process 7 channels. It would be limited to 5.1.2 as far as Atmos is concerned.
The X3600 has a better version of Audyssey than the X2600. The X3600 has MultEQ XT32 compared to the XT variant you'd get with the X2600.
THe X3600 is slighly more powerful than the X2600 and also includes pre outs for all channels. The X3600 also has 3 HDMI output, 2 mirrored and a third dedicated second HDMI zone output. The X2600 only has 2 mirrored HDMI outputs.
You'd not be able to have a 7.1.2 setup with the X2600 and if wanting such a setup with the X3600 then you'd not be left with any spare channels of amplification to power additional speakers in additional zones with. Yu'd need to use additional amplifiers or employ HEOS speakers if wanting 2 channel audio in other zones as well as a 7.1.2 setup in the main room. Only the X3600 would fascilitate you being able to have a 7.1.2 setup in the main room.
The 2600 is restricted to 7 channels of amplification and processing so a maximum of 7.1 or 5.1.2. The 3600 is capable of processing 11 channels with 9 out of the box. Both are capable of Zone 2 speaker placement. The 3600 has a superior room correction system in Audyssey XT32 over the basic XT. Your best receiver based on your requirements would be the 3600.