Use a 50 amp / 240 volt outlet for car charger and snow melting system?
Straight up, this is a hard sell

First, EVSE's require a dedicated circuit.  NEC 210.17.

Second, a 50A circuit can only serve one thing - NEC 210.23.  So it cannot serve a hardwired driveway melter and also an EVSE receptacle.  So that's right out.

Third, NEC 625.42 says the EVSE can be cord-and-plug connected if all are true:
1) it's part of a listed system
2) The receptacle &lt;= 50A
3) It's cord connected to facilitate a) ready removal for interchange, b) maintenance and repair, or c) repositioning of the EVSE
4) Cord length &lt;= 6 feet
5) Receptacles located to avoid physical damage to cord. 

I think item 3 will be a hard sell. 

So you're trying to argue that you should get an exception to the dedicated circuit rule (NEC 210.17); this outlet isn't an EVSE outlet -- it's a general purpose outlet that sometimes (wink, nudge) has a level 2 EVSE plugged into it.  Hmmmm.  That'll be an interesting sell to the AHJ.  AHJ==your local inspector; the one that you pull the permit from.  

The other prong of your argument, then, is that it's somehow reasonable for the driveway melter to be cord-and-plug-connected.  In practice, that means it comes to a junction box, which then has a cord or inlet.  This doesn't make a whole lot of sense, but it also absolutely requires the melter to call out a 50A breaker in its installation instructions.  If the melter is looking for a 20A-40A breaker, no deal because the breakers don't match. 

However, it would sell as a subpanel

If you convert this 50A circuit so it serves a subpanel instead of an EVSE, different deal.  The subpanel does not need a "main breaker" if it's in the same building as it's served from.  

In the subpanel you have a 50A breaker for the EVSE, and a ??A breaker for the driveway melter.  That solves the breaker size problem. 

Here's an interesting fact.  Nothing in Code prohibits you from misusing a "generator interlock kit" to interlock any two random branch-circuit breakers, just for the chuckles.  Only one can be on at a time. Go with either a Siemens/Murray panel or a Square D "QO".  Use Siemens' ECSBPK02 interlock ($30), or a Square D's QO2DTI interlock ($25). 

Now we open an ugly chapter: Provisioning.  The feeder cable must be provisioned to be able to suppor--- Oh, wait. You've got an interlock there, don't you? Well, then. Nevermind.