Though I myself have never tried that combination I might offer that you may have problems with severe cracking. Monel does not do well when diluted with high Cr (usually about 8% max)(Stellite 6 is 28% Cr). So you will have a high Cu alloy dilute into a high Cr alloy that is already crack sensitive due to its hardness. You may not even be able to stop it with low heat inputs or dilution. Cr and Cu don't like each other in large volume %'s.
It may be necessary to butter the Monel with a nickel alloy that accomodates the Cr and C in the Stellite a little better. Something without Cu. But even then you will end up with >5% Cu (~25% dilution) for a single pass. Thats high with Cr involved. So you may have to two pass it or use a more exotic process like strip electroslag to get your dilution down to the 5% to 10% range. And keep in mind, that a high Cr nickel alloy will experience the same problem with the the dilution of the Cu. NiCrFe alloys traditionally cannot tolerate much more than 15% Cu.
If your application can tolerate 'cross checking' (a stress relief cracking process) (common to hardfacing alloys) then you may be OK.