I would use a splitter and a small monitor mixer for the radar and a separate console for FOH. You can split before or after the preamps. If after, you just need to "y" the preamp outputs to both rigs. other than that it should work great. probably need a reasonably quiet location to put the radar rig.
Ditto the split before FOH. The last passive 24ch split I rented (before I built my own) was about $75 for the day. If you simply show up and try to work your setup, it's about a 99.95% chance the FOH engineer says "No." And you're out in the cold. Unless you're also running FOH all night.
The .05% chance of succeeding comes only if (1) you know the FOH guy, and he trusts you with his show, and is good with being two devices away from his mics. (2) you communicate your request in plenty of time to negotiate when he says "No." the first time. (3) he always wondered how 24 channels of API would sound through his PA rig. (4) he really doesn't mind your input gain decisions, and (5) he really doesn't mind mixing his show without redress if he actually needs a different gain structure in the middle of the show... 'cause you're sure not gonna want him with his hands on your APIs while you're recording it.
You have a 50/50 chance of bringing in a split if (1) the FOH guy has plenty of room for your rig. (2) he has plenty of notice and time to unhook and rewire the input panel of his desk. (3) he has plenty of time to check every channel of your snake for continuity and hum/buzz. (4) he trusts you to leave shared condenser phantom in his hands... and you trust him to leave shared phantom power in his hands. (5) you buy him several beers along the way.
I've run my passive on live tracking gigs in a couple of clubs here in NashVegas with no problems. I met each respective FOH guy a couple of days out, told him what I proposed to do, let him show me where he'd be comfortable with my rig living. Then I showed up early (a couple of times, at the first club, before he arrived to unlock), set up quickly, and we tested everything before soundcheck. After the first time in the second club, over a beer as we struck gear, the guy said "Hey... next time, drop your split off on Monday and I'll patch it in when I set up the room for your show." Boo-yah.
The FOH engineer will be hired by me that night so getting permission is not an issue.
I'm sure API and Neve pres will be better than the board. There wouldn't be any phanton power used. I hear what you're saying about splitters but I never trust them. The purpose of this is to get the best recording. The whole rig should fit in 1/2 rack. I would most likely place the setup to the side of the stage to get the shortest cable run possible. Not sure why the suggestion for a mixer? Why not use the FOH to monitor? I do this all the time in studio situations and it works great.
) (FOH needs to be able to mix) his show without redress if he actually needs a different gain structure in the middle of the show... 'cause you're sure not gonna want him with his hands on your APIs while you're recording it.
That's one excellent reason why.
Seriously, rent a good passive transformer splitter i.e. Jensen or Lundahl. One of you takes the transformer side, the other the direct side. Active splits like the Klark Teknik are fine too.
Then you let live sound do their thing while you do your thing.
Monitoring - you can't accurately monitor what you're recording over FOH because you will hear the sound of FOH and the room more than the sound the mic is capturing. Subtleties of mic placement and spill issues are mostly lost e.g. kick drums always tend to sound impressive over a big pa... even when what the mic is hearing is junk. Monitor off the Radar into a small desk of some kind (any kind really) and thru good closed back headphones or in-ears.
Plus you will almost certainly want to check your recording of sound-check while FOH is doing something else. FOH is not going to want to have to keep switching to your playback whenever you want to listen to something. That's assuming they're okay with 20 channels of line level signals in the first place...