Mikrotik route syntax to create a new routing table

I don't think you can do that.

TUN is routed (OSI layer 3), and it isn't possible to have two identical routes for a single subnet in this way (you would need at least some kind of difference in metric). TAP operates at OSI layer 2, where MAC lookups are possible, which is how your setup is working (when it works).

For TUN to work in this situation, your best bet might be to split your /24 into two /25 networks, and assign one side to each location.

You need to create static routes on each router so that they know how to reach each office's network.

On Office 1 router:
Create a route for destination network 192.168.88.0/24 with gateway the IP or interface of your VPN. (I don't know what TMG is and how it might reference to static routes, but the concept is the same regardless of the router vendor).
Since you mention some 'route relationship' (I guess this is a vendor specific terminology) you might not need to add any routes on the TMG router.

On Office 2 router:
You create a static route either via Winbox (IP > Router> Add) or via cli.

/ip route add dst-address=192.168.0.0/24 gateway=pptp-interface

Now Office 2 router know how to reach 192.168.0.0/24 (via the VPN) and likewise, Office 1 router should know how to reach 192.168.88.0/24.

Also I don't think you need any mangle rules. This can all be done without any iptables (IP>Firewall in MikroTik terminology) since it's just static routing between two networks.