how to level a second class/job on same character?

[Malusion]

Your most efficient path is going to be FATEs untill like.. 53? I haven't really worked out the best way to do HW levelling yet.

For a basic guide to the standard 1-60 if you want to go through it fast is:

1-7 - Hunting log, pick up any fates you see in passing.
8-14 - Western Thanalan FATEs. Leves, and leftover quests you have.
15-18/19 (Don't remember when optimal level to switch here is) - Western La Noscea FATEs
20-27 - South Shroud
28-33 - Eastern La Noscea. Focus on Costa side but try to convince your party to TP over to other side for Wineport FATEs too, you'll get through this bit nearly twice as fast if you do.
34-41 - Coerthas Central Highlands. Around this point you'll want to get into the habit of FATE hopping because the crowd is a lot bigger. Get a feel for how much you need for gold on a FATE (especially boss FATEs yield you no benefit to stay as long as possible) then head to others if you are seeing people starting them. The crowd is simply too big to stay controlled, and you'll miss out on FATEs entirely if you don't.
42-52 - Northern Thanalan. Because Heavensward FATEs are mostly terrible right now (expect 50+ to change in coming patches)
53-60 Dungeon. Lots and lots of dungeons.

Not the only way to do it, but it is the fastest.
Edit: And don't solo FATEs. Get a full on FATE group if you want to FATE grind, even if you're getting gold solo you're missing out on a lot of kill EXP from low damage and lack of tags. On that point, tagging mobs is significantly more useful to your group's (and thereby your own) exp than actually just hitting mobs in general. And having the map up on your screen zoomed out just to keep track of if people are doing multiple FATEs simultaneously is helpful too.