HI Bruce
quietly use two preamps through MENU 29 and 30 UHF-AMP and VHF-AMP power supply
The FT847 is not full duplex, so it will only feed one at a time and the current, at least with my LNAs, have had no problems.
There are also two things to know:
1 if you have an instrument or a PA after the radio, you risk that the power does not go through and short out before the instrument for its possible components on the line
2 when you activate the external one, the internal one can no longer be activated.
Personally, I preferred to feed separately after PA and pwr / swr METER, both because after the PA they cut off the power supply and in case of having 2 LNBs in series that can always be used, either one or the other or together, even if then it increases the NF considerably.
73
Franco
Frank