Is there a way to bridge two subnets, each having their own subnet router ?
Obviously each router need to have a proper route setup but short of that is there anything else to consider ?
Yes, you can do it with Tailscale. You just need to run tailscale with
--accept-routes --advertise-routes <your network here> on the nodes in the subnets.
We use it to connect to our VPC on Azure.
I guess I will still have both router set to have a static route redirecting packet for the remote subnet to the local subnet router ?
Yes that is correct.
This is referred to as site-to-site networking, which is a documented Tailscale use case.
How to arrange site-to-site networking with Tailscale is described in Tailscale’s Site-to-site Networking how-to guide, which states in part:
Use site-to-site layer 3 (L3) networking to connect two subnets on your Tailscale network with each other. The two subnets are each required to provide a subnet router but their devices do not need to install Tailscale.