I haven't RTFT.
Presuming both grooms are planning to wear suits (it's not a given) I think it could be fine. OP will know if they are wearing relatively traditional grooms' attire or not, or can ask. I read an article not long ago that full-length gowns for MoTB was becoming a thing, so why not MoTG? It was more of an American thing apparently, that has found its way over here.
It does rather depend on several factors, including:
- the overall style and formality (or not) of the wedding
- the time of day and location of the wedding
- whether it might overshadow whatever the grooms are wearing - it is their day and if they are understated it might be a bit much
-,whether it might cause confusion about who is who in the wedding party and how to handle this sensitively on the day or whether one or other groom is really shy or easily perturbed when stressed, and it would affect their day so it's best not to risk it)
- how both grooms would feel about you choosing such a dress*, based on how they want their day to be, how it will fit in with the other attire of guests, and so on
- whether it might clash, upstage or cause some other kerfuffle* with regard to what the other MotG is wearing or any groomsmaid/best woman.
*Yes you can wear what you want, of course, regardless. But, I think you come across as wanting to be sure on many levels. Or you wouldn't be asking Mumsnet.
I do like the dress, and it is nicest in cream, although I like it in red too and it depends on your colouring which others would be nicest on you. Is there a colour theme for wedding party for you to try to fit into? Or at least, not clash horribly with? It would look horrible on me as it's not the right shape and style for me. It could be perfect on you. So it's a definite 'maybe' from me 
.