Port forward a minecraft server but have 2 routers?

Both will have to be forwarded onwards or you could dmz your house one to avoid port forwarding both.

Log into Router Shop and go to the DMZ settings and put the WAN IP address of the Router Home there. Now router Shop is out of the way for port forwarding and triggering as all incoming connections are now being forwarded to router Router Home. Now in Router Home you can setup port forwarding & triggering rules just like normal.

Step 1. Login into your Router Home
Step 2. Find the status page that shows the WAN/Internet IP address and write it down. (The Gateway IP will be the IP needed to log into the Router Shop)
Step 3. Log into the Router Shop.
Step 4. Find the DMZ page
Step 5. Enter the IP you wrote down in Step 2 into the DMZ page and enable DMZ.
Step 6. Save and you're done.

You should setup a static WAN IP address for Router Home. If you leave it on DHCP the IP could change thus breaking the DMZ and of course breaking the port forwarding & triggering. This doesn't happen very often, but it is always a possibility.

First router has to forward all the ports to the second router's IP address (usually the WAN IP if it is chained off the first router). Then you port forward on the second router to the gaming PC.
Each port forward is one and only one step, to one device. If using multiple routers, you have to forward one step at a time, to the target device that is next in the chain only.