You would probably need switch (battery isolator, Dc-DC charger, etc ) of some sort to isolate the two systems to fully eliminate this, but part of the answer is the Lifepo4 keeps its voltage higher through most of it's capacity than lead does. Thus the voltage of a Lifepo4 at ~20% is the same as a lead acid at ~100%.

This is true, but in my case I use a DC-DC charger that does not allow the lead to charge the lithium (I don't want this or want to think about it, even if it was beneficial, in my case the complexity is not worth it) - the only way the voltage can travel is from the lithium to the lead.

Okay and you discharge the lead first and the lead only fully drains when all of the batteries fully drain.

Yes this is correct - but by not really discharging the lead, you will keep it alive for essentially forever.

How does this switching black magic work. Is there a power interruption while it changes the source power bank?

No switching, the system looks like this:

Solar -> Charge Controller -> Lithium -> DC-DC Charger -> Lead Acid -> Load

The load is always pulled from the lead acid, but in reality if the load increases your DC-DC charger will just put out more juice (until it hits its limits) and not really drawn down the lead. If you max out the DC-DC charger yeah you'll start to draw from the lead but that is ok - that is what it is there for.

For a really large bank that you need to draw down lots of amps you may need to rework things, but likely lead acid doesn't work for you in those cases anyway. This works for banks where your load is like 50 amps or less. For my case, I have 1kAH including the lithium and I do not ever draw more than 20 amps. Works great.

Thanks for the extra detail. I need to go dust off my Kirchoff rule books. :-)

You probably know more than me already, honestly!

Sorry if I was unclear - it is the other way around. The LiFePO4 is before the Lead. So it is:

Solar -> Charge Controllers -> LiFePO4 -> DC-DC Charger -> Lead -> Load

In this configuration, yes, the lithium is always charging the Lead, and the DC-DC charger is creating heat. But the DC-DC charger is not two ways. So the lithium does not get charged.

But the benefits are above, and also you can make the lithium bank a higher voltage bank (so 24v, or 48v, or whatever) which has benefits for your solar: you can use smaller wire.