Can I run 2.4.7 and 2.4.9 in parallel on the same domain?

No. Two Magento installations can’t share the same database (schema mismatches), same Redis (cache-key collisions), same OpenSearch (index conflicts), or same domain (cookie + session collisions). It’s a hard architectural no.

What you can do:

• Blue-green deployment. Run 2.4.9 on a parallel staging environment with cloned production data. Smoke-test, get UAT-clean, then cut DNS over at a planned maintenance window. The two never serve customer traffic simultaneously, the cutover is atomic.
• Subdomain split. If you have two separate storefronts (e.g. shop.example.com and b2b.example.com) on truly separate Magento installs, each can be on a different version. This is rare but valid, usually for staged migrations where one brand goes first.
• A/B testing the upgrade. Not supported. Magento doesn’t have a built-in mechanism for serving 1% of traffic to the upgraded version. You’d need an edge-layer router (Cloudflare Workers, Fastly VCL) doing the split, doable but expensive.

The blue-green pattern is what I recommend for stores above $2M annual GMV. It costs roughly 2x hosting for the ~1 week of overlap, but eliminates downtime risk and gives you an instant rollback path. For stores under $2M GMV, in-place upgrade in a planned 30min, 4h maintenance window is fine.