Personal augmentation


Can a Muscular Biding (CSC 89) and Physical Augmentation (CSC 89) be used togeather?
The first one adds +1 to STR and DEX for 250 000. (TL13)
The second one adds +1 for 500 000 (TL11) ,+2 for 1 MCr (TL12) ,+3 for 5 MCr (TL15) to STR or DEX or END.
So if you want to have +2 to STR and +2 to DEX you can combine two and spend 1,25 Mcr rather than buy higher version of Physical Augment and it saves you sweet 750 000 Cr.
Is it correct or am I missing something? Or is there any restriction that you can only have one source of augment for a specific atribute? Or other limitations?
There is no generic rule. Read the description and make up your own mind. All augments don't have to be available in all campaigns.

I would not allow stacking the same augment several times, otherwise I would allow it.

All augments don't have to be balanced, some are better than others, or perhaps lower tech? E.g. Muscular Bridging is TL13, Physical Augmentation +1 is TL11. If you have access to both, start with Muscular Bridging?
Another question. Is there a rule about installing multiple augments in the same time? Or you need to install one by one because the body cannot withstand the installation of several implants at once?
The short answer with cybernetics is "nope, there really aren't any definite rules". Transhumanism in general, including cybernetics is not really a focus of the Traveller universe. They've been added in recent editions, but they don't have the kind of robust detailed support that you find in games that are actually about such things. It's pretty much "if you want to use cybernetics, here's some general guidelines." They don't have rules that pin the cyberware into a particular default. What works best for your campaign?
By TL 12, installing several at once will probably not be an issue due to how advanced the medical technology has become, and I suspect that it would be the same even by TL 10 or 11. It may require a bit longer a recovery time, but that would be as much due to the person needing to get used to all the new parts as any issue with attaching or implanting them. Being able to replace multiple things at once may also be necessary if the person sustained what would have otherwise been lethal or near-lethal injuries.
I like the way Cepheus Deluxe limits cybernetic augmentation, where each addition has a price in terms of a loss of a certain amount of humanity in the character, and once you get over a certain level you can no longer think like a human anymore.

Probably what happened to Darth Vader. Although he was already going a bit nuts before the cybernetics took over lol.
Probably longer recovery times at the least. If it takes say 1D+6 weeks to recover from a single augment installation, add another 1 D weeks per additional augment. You could add DMs based on the PC’s END, medical TL, etc.