Why are there two replicase proteins translated from tobacco mosaic virus RNA?
Very Short Answer
As of 2021 it does not appear to be known why Tobacco Mosaic Virus (TMV) produces two proteins from its replicase gene.
Relatively Short Answer
I have done a brief literature search on the topic, and found papers from 2008, and 2012 which relate to the replicase and studiously avoid the question of why there is a 126-kDa and 183-kDa form of the TMV RNA-dependent RNA replicase. I am therefore of the view that there has been no advance since the review by Buck (Phil. Trans. R. Soc. Lond. B (1999) 354, 613–627). From this I glean the following:

“…both the TMV-encoded 126-kDa and 183-kDa proteins are required for maximum efficiency of TMV RNA replication”. The full-length protein is absolutely required; preventing the production of the smaller protein reduces the replication rate to 20% of normal.
“The read-through portion of the 183-kDa protein contains amino-acid motifs characteristic of RNA- dependent RNA polymerases (RdRps) and hence the 183-kDa protein is likely to provide the catalytic activity for the synthesis of TMV RNA from NTP substrates.”
“The C-terminal domain of the TMV 126-kDa protein is helicase-like. Although helicase activity has not yet been demonstrated for the 126-kDa protein of any tobamovirus, these proteins contain six amino-acid motifs, which are highly conserved in known helicases, such as the translational initiation factor eIF-4A.”
“The TMV 126-kDa protein contains two domains. An N-terminal domain with amino-acid motifs and predicted secondary structure typical of S-adenosylmethionine binding proteins, methyltransferases and guanylyltrans- ferases is probably required for synthesis of the 5' m7GpppG cap structure. The TMV 126-kDa protein has been shown to have guanylyltransferase activity.”
There is some evidence in other viruses for association of two forms of replicase. However this is indirect, and there are no data for TMV. The implication in the original wording of the question that the two forms in TMV were two subunits of a replicase is not supported by any evidence and is not assumed to be the case.

The only suggestion that Buck makes is that as helicase activity is required for two separate purposes — to unwind double-stransded RNA during replication, and to remove secondary structure from single-stranded RNA when this is used as a template for multiple copies of the complementary strand —  the two forms of the replicase may have a different substrate specificity in their helicase activity.
[Updates welcome — I may have missed something recent.]