This is sort of why I want to combine them, and save art forgery for people with the proper art proficiencies.
I get what
@Ovinomancer is going for, but my experience with artists just makes me see forging as a sub-skill of the main tool set. For example, there is a channel I watch called Drawfee. Great people, great art. Especially a lady by the name of Julia, she is just amazing. And they did a series of videos where they copied styles, either each other's style or the style of famous artists, and they talked through the process and some of them were incredibly close.
Look at fanart sites, where people copy the designs of animated characters. Many of the best artists make stuff that looks nearly identical to the original work.
And, while I understand the idea that the Calligrapher and the Forger have a different approach, I would find it weird to have a forgery proficiency and that be the one for all writing. Because the Calligraphy set in Xanathar's does give a few uses that are interesting for the game, and forgery is a subset of that.
Actually, reading forgery, here is an odd thing. Forgery specifically calls out combining it with other tool sets. Like if you had cartographers and forgery tools, you can make a fake map. But there is no talk of fake maps with Cartographer's tools. But, which skill by itself is more likely to let you make a fake map? Knowledge of how to copy another person's handwriting, or knowledge of making maps?
I don't think Calligraphry can fully stand on it's own, but forgery definetly can't stand on it's own. To forge a painting you need painting tools, to forge a statue, you need knowledge of mason's tools, Woodcarvers (or carpenty) tools or blacksmith's tools. To forge a ceramic pot, you need knowledge of Potter's tools.
The only forgery that you seem to be able to do solely with forger's tools is writing, which is the realm of Calligrapher's tools. So, I would just melt them together and call it day, because tools profs tend to be broad anyways. Just have Forgery be a high DC version of any of the normal skill profs.