2 different Time Machine set ups on one external drive? How to keep both and run correctly.

My situation is that I'm worried about to lose my one and only Time Machine Back up.

One and only one Time Machine backup does not comprise a robust backup strategy. You must never, ever be concerned about losing either the contents of the source volume or the contents of any particular backup. Not only can any device can fail at any time, a single catastrophic event or even an oversight at a critical stage can result in the simultaneous loss of all data. For an example read this recent Discussion.

The whole goal is now to make a second back up.

In that case, obtain an additional backup drive and dedicate it exclusively to Time Machine. Time Machine will back up to as many additional backup drives as you wish to provide.

Is there a way to secure my only back up somewhere else just in case?

If you mean transferring it to another device the answer is no. We may no longer move Time Machine backups from one disk to another, and the procedure once described in an Apple Support document has been removed. If you value that backup, set that device aside for use only in the event you have a need to restore backed up items.

But when I back up on "MacOS" the Format will change from:

Mac OS Extended (Journaled) to APFS (Case-sensitive) automatically by Time Machine.

That is normal, and how Time Machine works these days.

Summary: Buy an additional backup drive. Consider buying two. If all backup devices are to remain in the same room, office or dwelling, then buy a third. Keep at least one of them geographically separate from all others at all times, and you will have an effective and robust backup data recovery strategy as immune as practicable from catastrophic, totally unrecoverable data loss.

John Galt wrote:

Summary: Buy an additional backup drive. Consider buying two. If all backup devices are to remain in the same room, office or dwelling, then buy a third. Keep at least one of them geographically separate from all others at all times, and you will have an effective and robust backup data recovery strategy as immune as practicable from catastrophic, totally unrecoverable data loss.

To the OP, adding to what John Galt has written…

And if you’re sharing this backup with multiple Macs, use network-attached storage with Time Machine support.

I probably wouldn’t go with an SSD for Time Machine either, unless your windows for performing backups are very short, or unless you are frequently restoring files or whole Macs. HDD capacities are far cheaper, and waiting for a backup to an HDD usually isn’t an issue. (And if I had the budget for capacious SSDs for backup, I’d still lean toward NAS; Synology or ilk.)

For backup sizing, go at least twice and preferably closer to three or maybe four times the size of the input volume. Lower for less file churn, higher multipliers for higher churn and for larger file sizes. TM really doesn’t like running out of room. For scaling, for terabyte-scale Macs, I’m using four to six terabyte HDDs and depending on price and availability, or using NAS.

SSDs are preferable to hard disk drives but their singular advantage is speed, which is utterly irrelevant with Time Machine. Doubly so with a NAS, whose data throughput will be limited by the network connection.

Redundancy and simplicity are desirable elements of any backup strategy. You appear to have been satisfied with a barely adequate 1 TB Time Machine partition, but a 2 TB hard disk drive costs about $60 — perhaps $10 more than a 1 TB HDD. For the price of one SSD you can buy two cheap, slow, disposable HDDs of similar capacity.

Yes. Time Machine is a background task specifically intended not to monopolize a Mac and burden its CPU so that it can work quickly. In fact if you are actively doing something particularly demanding such as rendering video it will pause and do nothing until those more demanding tasks are complete. There is so much processing "overhead" involved that the read / write speed advantage SSDs offer is totally lost.

My pleasure. Glad to help.