I don't think you can balance redirects. If the purpose of this deployment is to redirect all traffic to these servers from http to https, you'll want to listen on two separate IPs, and redirect accordingly. This will mean that the user must know the subdomains in advance.
What you're attempting to do strikes me as not so good. A user is going to hit your frontend URL, and be redirected to one of two subdomains. If they return, they may be redirected to the other subdomain. As a user, I'd find this extremely awkward.
The purpose of load balancing, is obscuring the implementation details of how many and what servers are in the cluster, and provide a uniform front end.
I'd suggest something along the lines of the following:
Frontend: 10.80.11.1:80 # http://blah.com/
Backend 1: 10.80.11.5:443
Backend 2: 10.80.11.6:443
In each of the backends, have their default page load at
/gohere rather than trying to get HA Proxy to rewrite the URL. Now you can load balance all you want, obscure the details from the user.
If you REALLY want to achieve the rewriting of the subdomains, you could attempt something like this:
Frontend: 10.80.11.1:80 # http://blah.com
Backend: server 10.80.11.2:80
Backend: server 10.80.11.3:80
Frontend: 10.80.11.2:80 # http://ssl1.blah.com
redirect location https://ss1.blah.com
Frontend: 10.80.11.3:80 # http://ssl2.blah.com
redirect location https://ssl2.blah.com
However, you'd need to install HA Proxy on all 3 hosts (unless any of the above backends share a host and have multiple NICs).
If you're using Apache as the backends, you can achieve https redirects very very simply and shouldn't manage this with HA Proxy.