ORBI, Cost effective expansion - 2 RBK53s?

Yes, of course, you can use the satellites from one RBK53 set with the router from an another.  Although satellites are packaged in a box with a router, they do not "pair" with the router until you physically do so. (One at a time.)  At Amazon's current pricing (even better at Costco?), it is actually less expensive to purchase an RBK53 package than two RBS50 satellites, so the extra router is "free".

I do use VPN, so I am not competent to advise on whether you benefit from having one Orbi to handle your entire house WiFi and another to handle your office by setting up a VPN.