I had leftover 13 eur on my Revolut so I tested it. I used my own Revolut card to top up own Monzo card and it worked. On Monzo it just shows as top-up, Revolut shows it as payment.
I compared rates, for sending 10 GBP from EUR:
Revolut currently shows 1 eur = £0.9210 (which will settle later, I’ll update)
Transferwise gives rate 1 eur = £0.92540, but also 2 euro fee, which is rather high for £10, but then it can be really low for larger sums of money.
Transferwise is kinda convenient, because one you set up both accounts (I coincidentally explained it here earlier today, shameless plug), it’s very easy to send money over and over again. And you can initiate payment from debit card directly, which saves the hassle of moving money between accounts.
Transferwise has fees but very reasonable, Revolut has no fee up to £5,000, and 0.5% thereafter.
Might be worth checking both, as Revolut/Prepaid Monzo can be hit by GPS outages, while Transferwise is more independent (I think). With Monzo current account already being slowly rolled out and GPS not being a problem ever again, Current Account + Transferwise personally seems like more stable combination.