Runes of Magic US / AU

I recommend staying within the 10 level difference as all quest I know of require to be within 10 levels to turn in. I usually advance the higher side to open the higher level quest, then turn in on lower side. This works well for dailys. Most dailys in coast of opportunity and xavieria, then next is really the rorazan and up are good for this.

In addition is goblin mines with a friend, or dreamland.

There is also xp events to get the lower side up.

if you are leveling both classes using quests at the same lvl as your classes, than you should get 2 lvls on a class before switching to the other class. this way you won't have to keep going to the housekeeper to switch classes to be able to accept the quests.

alternatively, you can run all the mainline quests (ONLY the mainline quests) on your main and get to max lvl and than switch to 2nd class and get the rest of the quests (ofc you might run out of quests to lvl up if you do it without an xp event)

The best results I got were when I stayed within 5 levels. To allow more than a 10 level difference if you intend to use both classes is usually a disaster because they have to operate in different regions with different sets of gear. As long as you stay with in the 10 level difference you can do the quests and kills with your stronger class and get the xp and tp for the strong class that way and then turn in the quests with the weaker class and get the xp and tp for the weak class that way. That system works very well and will usually keep your classes at near the same level.
Being close to the same level usually within 5 levels also has the advantage of being able to use one set of gear if your classes use the same type of gear which is a great saving of time and money, Also if your strong class uses a lower type of gear cloth or leather your other class can usually survive using that gear since it is not doing the killing.

You could save some quests when there is an EXP event to level your second class. Since only a few skills will actually be used from your secondary, depending on what your secondary class is, 70 is usually the highest people level the secondary classes to. If your secondary is something like a warden, i would max it out for Briar shield to be maxed

But overall, my recommendation is to save quests for your secondary and do them during an EXP event, thats probably the best F2P option in my opinion. If you are P2W, just buy several hundred Daily Reset Tickets and it should be an "Easier" power level.

The skills you need to focus on depend on your class. I know very few people who have everything maxed out. you have to make choices. a general rule of tumb would be to max out everything that maxes at 50 and all the buffs then try to get your rotation maxed out.

As for the best way to level 2 classes i would say do quests on your main class, then do dailies on your secondary class. you can make it go faster by doing higher level dailies on your main class then turning them in on your off class. you can turn in quests up to 10 levels higher then the class you are using, but can only accept quests 2 level higher then yours.

For skill focusing, there are lots of methods to gain more tp than the normal rate of 1 tp for every 10 xp. Try and look up some of those or ask around. For instance using the guild tp buff tower and logging off/afk in your house with furniture that gives a tp bonus.

Normally at the 1:10 ratio there is enough tp to keep about 3 skills maxed as your character gains levels. Originally you can keep more, but that last to somewhere in the 40 level range. At 50/51 there is a jump in tp required to level skills. It goes from around 50k to 100k per a level, and the cost keeps going up from there. iirc it is about 2.4mil tp to go from 94 to 95.
edit: Also you may not want to keep a few skills maxed, but instead keep more skills at higher level. Instead of 3 skills at 95 do 1 skill at 95 with 6 skills at 85.

I am slow and did not get a feel for which skills are most suited for a well rounded rotation until my character was in early 70s. You should have all skills unlocked by then including the elites. I recommend saving the advanced skill reset stone you get from level 30 bag and use it much later, or save it for a class balance patch.