WOW, by looking at that manifold you are trying to buy I can tell you one thing SLOW DOWN, get expirience, dive with people more expirienced than yourself, I dont know anyone who is or would use a manifold like that anymore, much less that manifold may not/probably not fit the current threading on tanks today.
I would use that manifold if I didn't already have one that is quite similar. The center outlet is perfect for a double hose regulator. Isolation was never a consideration and the J feature is optional if an SPG is attached. Since each tank outlet has a dedicated burst disk, I wonder if that manifold has a port for an SPG? It certainly looks like it.
My guess is that that specific manifold is designed for 6.9" diameter tanks like the Al 50 or the old LP 72. It appears to have 3/4" threads so that shouldn't be a problem. I wouldn't bet on it working with any modern tanks of 7.25" or 8" diameter. The manifold, bands and tanks all need to match.
My specific manifold also has a post valve on one end in addition to the center outlet. By orienting the manifold backwards, the post winds up on the left side where a first stage can be mounted and used to provide LP inflation and an octo. But still no isolation...
If the goal of doubles is, as the OP states, very long but shallow dives with no overhead (physical or virtual) then all the isolation and double regulators just isn't an absolute requirement. In effect, this type of manifold does one thing: it increases gas capacity. The diver still has the same single regulator they would have if they bought a HP 250 (if one were made). We see divers all the time buying HP 130s because Al 80s are too small, so what's the difference? It's still a single tank with a single outlet for a single regulator. No more or less redundancy than any other OW recreational diver with a single tank and regulator.
The real problem is tank selection. It would be better to use tanks that aren't highly negative in that a wing failure might be survivable. But this is an entirely different discussion.
Now, just for the record, I don't recommend doubles. I can't carry the weight and I don't do the types of dives that require either the redundancy or the volume. But, if I were building up a set of doubles with something like HP 120s, I would use a proper manifold (and this isn't it!) with double regulators, a high lift wing and probably a drysuit for redundant buoyancy.
I also think that doubles are a topic for divers with well over 100 dives. I know, someone will chime in with the fact they did their OW checkout dives with doubles but I still think it is an advanced topic. Not the kind of thing a beginning diver needs to get involved with.
So, yes, I would dive that manifold. But, no, I don't recommend it unless it used with a double hose regulator. Double hose regulators are a lot more fun than single hose.
Richard