Best way to Sous vide two steaks to different temperatures

Pretty much...yes, but you can fix it!.

When you properly sous vide or very slow cook anything, you'll retain more of the myoglobin color because of the even cooking that often doesn't go above 140 at all. So a properly cooked steak like this will retain much more of its red colored myoglobin. Simply put, the meat will have more red juices to release! (Its a great, great thing about sous vide.)

As @Ronald mentions, the other thing is the resting of the meat. It's an important step as the muscle fibers relax after the heat is off and hold juices better then. An often misstep for the home cook is they allow a hot piece of meat to rest on a flat, solid surface. This causes the bottom of the meat to steam against the board, open the fibers in the meat more, and release the juices on to the board. Rest your meat on a raised baking rack so that it has air circulation all around it.

After a short rest - for most steak 10 minutes is fine, then you can cut into the steak. Use a very sharp knife to slice. The meat here is essentially a sponge and you don't want to compress it and squeeze out the juices. A dull knife will do this and you'll lose more juice on the cutting board again. Use a sharp knife and apply steady, even, but light pressure while slicing - let the edge do the work (if it won't, sharpen the knife more).

The machine

I invited a couple of friends who are keen to try sous vide over during the whole process. All 4 of us agreed. The machine is going back.

My immediate first thought was that you had a bad machine. The Vac-Star Sous Vide Chef has excellent reviews on Vac-Star's website and it looks like you are using it within proper specifications. That would be my first hunch, as nothing else seems particularly out of place. If you are able to come back, since this was over a year ago, it would be curious to see if a different machine solved your problems.

_{On a side note, I looked up reviews for Sous Vide Supreme vacuum pouches and they seem good, too.}

Other things

I looked up for reviews of the I noticed the Beyond Salmon blogger had posted a follow-up experiment with sous-vide steaks that I found had useful data. This summarized it well:

What most sous-vide books and websites tell you is that you can't overcook using the sous-vide method. It all depends on how you define "overcook." The steak will not go above the desired temperature no matter how long you hold it in the water bath (well, dah!), but the longer you hold it, the more juice you lose.

I have noticed this from your post:

I have tried various temperatures and times from 55°C to 60°C and from 40 minutes to 4 hours.

Per her experiment, steak A, B and C turned out the best. They also had the lowest temperatures and cooking times:

Steak A: 121°F (49.5°C) water bath for 50 minutes

Steak B: 126°F (52.2°C) water bath for 50 minutes

Steak C: 131°F (55°C) water bath for 100 minutes

It likely would be a good idea to try lower temperatures, and stick to the lower-end of the time scale. Also from the article:

But if you are using an immersion circulator, you might be tempted to put meat in the water bath whenever it's convenient (in the morning before going to work, during kids' nap, etc.) and then have it ready for dinner. It will surely be very tender, but you'd better have a lot of demi-glace handy because it will be dry.

Hope this helps!