welcome Poppie.
I dont think you can really. in theory it should be possible but grafts are usually on woody stock but tomatoes are annual so dont produce the internal structures for the graft to take.
It is a lovely idea. perhaps comercial growers could but i havent seen any.
quite happy to be proved wrong though.

I hate to disagree with Seaburngirl but as tomatoes are commonly grafted on to disease resistant rootstock, it must be possible to graft two or more different varieties onto the same rootstock.
I guess you would need to leave the rootstock plant to grow until there are two main stems, and then graft your separate varieties on to each. Grafting is done with proximity grafts in which you remove a slither of the cambium layer from each stem and then tie or tape them tightly together, leaving each growing in its pot until the graft has taken.
Personally I wouldn't think it is worth all the trouble. Grafting to overcome the problems of persistent diseases in the soil is worth the trouble but why not just grow the two different plants close together. You could even grow them in the same pot or planting hole and simply intertwine them. As they're an annual there's nothing to be gained from putting two on a single plant apart from the challenge of showing you can do it.

dont mind being corrected at all. is this in commercial greenhouses then Bertie as I dont see the point in the private gh? certainly the cambial layer produces the immature 'stem cells' protoxylem and protophloem that need to be in contact as you say.

I have actually done this many many moons ago as part of a Rural studies course at Padgate College in Warrington. Ah what a fun course that was. Agree with Seaburngirl, what is the point? We only did it on the course to learn how to do that kind of graft rather than anything else.

Sorry for delay in replying, been away for a few days.... I think the grafting to a disease resistant rootstock used to be done commercially where diseases had built up in the soil and it was easier than changing the beds or growing with ring culture or straw bales. Nowadays it's pretty much all hydroponics anyway, so there's no need.
Amateurs might do it if their soil is affected after years of growing tomatoes in the same soil, but that would be a rather lazy approach as changing the soil in the beds is probably more effective.
If you grow your own plants from seed I doubt you'd ever have problems with soil diseases anyway, and we certainly never have had despite growing toms in the same spot every year.