The Day of the truth is there
I conected both Motu UL mk5 with USB to an PI 5 with CamillaDSP Software.
Build up the Alsa Device and made that /etc/asound.conf with 36 Channels at all.
Then i configured CamillaDSP.
And made the Measurements between the Channel Out One, on both Devices.
Timedifferences is 462us, even if you conect both devices with Toslink and sync the second Device to Optical IN, this Timedelay will be the same!
But:
Without the Optical Sync between the two Motus you get an Drift of that Delay Time, it is Moving over Time like an Slowmotion Movie on Osciloscope.

With Sync it is Stable at 462us
Even i switch of the Input and start it again the Delay is fixed at 462us.
On first Motu i measured the Time Betwen Channel Out 1 and 2. Timedelay on Scope was around 0-2us.
After i add an fixed Delay to Motu 1 Out 1 of 462us
I got also Timedelay, between Both Motus Out 1, of 0-2 us fixed!!!
So now i need to Check all chanels in between.
And of course making an Settup with Speaker and Check how it sounds
Checked it between Motu 1 Channel Out 1 and Motu 2 Channel Out 10 the Delay is the same. after adding 0.462ms to Motu 1 Channel Out 10,
Both Channel 10 on Motu 1 and 2 had 0-2us again
It looks like CamillaDSP Channel 1-18 is now Motu 2 ((hw:CARD=UltraLitemk5_1,DEV=0) and Channel 19-36 is on Motu 1 (hw:CARD=UltraLitemk5,DEV=0)
Not sure why.
After reboot the Motu 1 was again Channel 1-18 and Motu 2 was 19-36 !
Need to watch at this may be some Timing problems at boot for USB Detection not sure.
I changed also the Buffer on the Motus from 512 to 256 and the Delay after Boot was at 362us between both Motus on CamillaDSP.
I guess 400us isn't at all an Problem because some Systems could set A/V sync between 0 and 300ms.
I read some People using 60-80ms (Lipsync, A/V sync ???) on an Surroundreciver, so not an Brainer i guess.
But need to check it with aplied FIR Filter by sure.
New Problem found and can't realy see why.
If i use 1khz Tone to detect the Delay between the Channels i see that 364us Phaseshift Between Channel Out on Motu 1 and Motu 2.
1 add an Delay to Motu 1 with 364us on Camilla and the Phaseshift on 1 kHz is fine at 0-1us.
Now i change The Inputfrequenz to 5 kHz and get an new Phaseshift of 41.5us

now i am a bit confused.
And at 100Hz 1.06ms

thats realy Bad but why?
I used REW Generator to feed in the Tone from HDMI L+R on Windows to "HDMI to I2S" Board on RPI 5
And between OUt 1 and 2 on Motu 1 no Phaseshift, anyway what kind of Frequenz i Use how suggested.
Probably not the right way to make it.
I go now for the 16 Channel Setup with adding the Motu 2 thru LWL (4 Channels) and SPDIF (2Channels) instead of 2x USB
And do the Test again.
Robert