Azure Express Route Question

Hi There, Have been doing a bit of reading around Express Route/BGP etc, I am familiar with Azure/IaaS/Wintel in general but not so much Networking I know basic networking about BGP, user defined routes in azure etc.

My Questions is,

If I have 2x ER Circuits, say they're both 1GB links, and both going back to the same on-premises network. I understand the routers will be advertised via BGP.

I've had a look around and i can't figure out how i direct certain traffic down one of the routes, and other traffic down the other. I'd like to do this at a subnet level

Say I want all traffic in Azure to use one of the circuits, APART from a couple of servers in Azure relating to a web application, i want those 2 servers to be the only thing going down the second route.

on the Azure side how do i direct traffic back on-premises for those two servers, telling them to use only circuit number 2.

Is this done via some BGP magic I need our network guys to do, user defined routes, as in i can some how route the traffic of those servers (in their own subnet) using a UDR, or something else?

Please forgive me if i've missed it, but i've read a bit of MS doco on ER and Routing/BGP/Peering etc but its still not clear to me. I've done alot of googling and see people asking the same question but no solid answers

thanks in advance!