Are there any 2 things I can do simultaneously?

[Alxndr]

Like woodcutting + something
or combat + something
etc?

Or is it always just one thing and one thing only at a time?

[VectorX]

Passive skills like Township and Farming can be done at all times together with other skills since they're, well, passive.
The rest can be done only one at a time. There are probably mods that allow multiple skill training but I'm not sure.

[sonen4]

To add on to what was already said...

Summoning can be trained with any other skill, because you earn summoning XP both by creating tablets within summoning and by equipping and using tablets in any other skill.

There are a few other niche ways to train multiple skills:
1/ There is a special area in fishing that can only be reached by equipping the barbarian fishing gloves, and all the fish there give both fishing and strength XP.
2/ The Devil + Ent familiar synergy allows for training both woodcutting and firemaking at the same time.
3/ The Flaming Axe Scroll, which is found in township trading, also allows both woodcutting and firemaking to be trained at the same time.

[VectorX]

Originally posted by sonen4:

To add on to what was already said...

Summoning can be trained with any other skill, because you earn summoning XP both by creating tablets within summoning and by equipping and using tablets in any other skill.

There are a few other niche ways to train multiple skills:
1/ There is a special area in fishing that can only be reached by equipping the barbarian fishing gloves, and all the fish there give both fishing and strength XP.
2/ The Devil + Ent familiar synergy allows for training both woodcutting and firemaking at the same time.
3/ The Flaming Axe Scroll, which is found in township trading, also allows both woodcutting and firemaking to be trained at the same time.

Right, forgot about Summoning gaining XP while using the tablets in other skills. I guess that also counts as kinda passive.