good idea, i will connect some monitors, and update about the results. That indeed takes away already some errors that might be made by the input cards of the second pc.

Wow, thanks Mathijs! Very nice you tested this for me!

I also did some testing this weekend, while we were on a gig, with 2 monitors, and timelapsed pics to get the results. Most of the time they were in sync, but sometimes it was 1 frame off.

Now the challenge is to find another decklink sdi card, since it looks like they're not in stock anymore and the second hand ones all appear to be sold last week :(

I could buy 2 x decklink duo, or 2 x decklink sdi 4k, or risk it to mix a decklink sdi with decklink sdi 4k and hope there will be no driver issues.

What device did you use to generate the sync? I was thinking to buy the blackmagic sync generator.

Indeed, the preview and program were off by one frame. The external sync locks the frames of the cards together, but if the software frame-copies preview one frame before program to the decklink output, it won't get that in sync.
Only thing is, I do not think this would be a problem. Explain why you think it would be please?

I use a Grassvalley ADVC G4. It costs 3 times what the BMD costs, but it takes external sync, so it can for example sync to a OB truck's clock.
It has multiple outputs that are configurable in HD and SD, so it can sync mixed sources.
The BMD sync generator can clock your gear, but cannot sync to another system. So a lot of times it will be useless.

For this job you have to do, you do not need a sync generator at all when you get the decklink duo and get a sync signal from the OB truck. It goes into the Declink Duo and there won't be any sync problems between you and them that way.

I just finished a long torture test on my system, using 2 x decklink sdi and a blackmagic sync generator on the outputs. I used 5 camera's and a lot of videos, to make the system work as hard as it can.

After a lot of help from Mathijs in private messages, I could get my outputs 100% locked together! Thanks Mathijs, you rock!

The trick is to put the blackmagic sync generator as reference input into the cards. Once the 'locked' status is shown in the blackmagic desktop video software, they will not drift away from each other anymore. If there is a small delay between the 2 outputs, i can now simply drag the slider in the blackmagic software until they match. It could be for example that there is 1 frame difference between the 2 outputs, but since you can adjust for that, it's no problem at all.

Right now i'm dropping 3 frames per hour on both outputs, but they drop perfectly in sync, and it's 'due to source framerate'.

One strange thing I noticed: even with no sources in vmix (2 empty inputs) I'm dropping frames 'due to source framerate). I'm wondering what could be the cause, and if I'm really dropping any frames at all :) Not that it's bothering anyway, they drop in sync, so problem solved!