Hi , if you want to use second session as backup only , you can configure Conditional Advertisement Enabling Conditional Installation of Prefixes.
This will allow you to announce/receive prefixes to second bgp neigbour only if some tested prefix (for example 0.0.0.0/0 ) received from primary peer is missing.
Juniper Business Use Only
Hmm, that is VERY bad idea.
Thate are 2 options for you
Why you can not use lag ?
1 - why can't you run STP on the CE switch?
2 - for multi-homed CE like yours, a nice capability of EVPN is supposed to allow for active/active forwarding and deal with loop prevention at the PE