1. This methode will only work, if both thermo elements can be held at the same temperature!

2. A better approach is to use a high quality relay, where both contacts are made of same material. Such relays are often made of heavy gold contacts resting in an inert gas atmosphere.

ad 1) two contacts in the same realy may have a very samll difference in temperature; however, you will cancel the Seebeck-effect of the coolest and the resulting error will be saveral orders of ,magnitude less than just using one contact.

ad 2) "better approach", sure, but soo much costlier. If you can reduce an error of min 100uV (one contact) to max 1uV (two contacts in reverse) that will suffice in 99.17% of the cases.

Erik