Sorry to see that.

1. It's not a lithops, that is a Pleiospilos nelii aka. splitting stone

2. It belongs to the mesembs family of succulents, this type of plants have very different watering regime, and is generally not recommended to be planted together with other types of succulents. Especially in this case the aloe and crassula both need significantly more watering than the pleiospilos.

3. From the first picture, the plant is going through the splitting phase, which means it is generating new leaves (the small ones in the middle) and it is taking all the water/nutrients from the old leaves (the plump ones). During this period NO WATER should be given... so even a bit is too much already. Only start to water lightly after the old leaves are completely gone or dried up. When these plants are overwatered, they either rot, or explode. Looks like yours had a little bit of both here...

4. Don't give up! Read up more information on these things before you try again. But separate pot for these are highly recommended.

Here's mine, also finishing up consuming the old leaves, while spitting out the flower bud. I don't dare to flower it right now either.

Thanks!!! It is probably a good start to know what plant I have. Both ones I have tried with have been in pots with other plants, so I think i will try once more and give it it's own space.

Thanks for the advice!!

Yeah, even grouping more than one of these together could be tricky, cuz if one of them is splitting and the others are not, then watering becomes complicated again. So that's why I only dare putting one in its own pot. Mine is a 3" terracotta clay pot, it's suitable for mesembs because it dries out faster.

suivezmoi,
Why don't you read something from our "resident expert" - he grows them by dozens, and very well (and few per pot too).
One of the threads, for example (there is more if you search GW):

http://forums.gardenweb.com/forums/load/cacti/msg0300041412368.html

or from Steve Hammer - here you can really learn a lot: https://files.nyu.edu/ms689/public/AGM.html