I am wondering if I should get an iMacPro Lap Top and use my desk top old iMac SCREEN for a larger screen option. You, Neal and Joe all suggested this to me originally and at first I was against it. Now I am wondering if this whole issue with wrong computer ordered is a sign for me to go this route.

I would only want to get a Mac Lap Top IF AND ONLY IF I can use my big screen on old iMac - Is this possible? Or is my Desktop iMac old one going to die soon and that is not going to be an option for long.
I was trying to figure this out myself and found this article but still not sure of answer.

https://www.macworld.com/article/3532366/add-a-second-display-to-your-mac.html

Can you help me?
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Using your iMac as a second display is a massive massive headache. It is a miserable experience and I would rather walk over hot coals than to deal with what it takes to make it work. Here's why:

1. You can only use a very specific year range of the old iMacs. It doesn't work for all iMacs.
2. Apple will eventually stop support for doing this with a future update just because they don't care about this feature.
3. You actually have to have an active user running on the old iMac and logged in so it's not really just a display. It's a user running that you can then basically push your video over to.
4. It doesn't work half the time and the other half of the time it works for about 5 minutes.

Short Answer: Forget about it. You are better off buying a second display and just using that. You can go cheap like around $150 or kick it up a little more to around $300 and get a really nice display.