When casting a spell, how can a mana ability be paid with another mana ability?

Question

A situation was presented with Skyshroud Elf and Ashnod's Altar: apparently, you could generate WW or RR by:

• Activate the Elf's filter ability ({1}: Add {R} or {W})
• Activate the Elf's filter ability
• Activate Altar to generate {2}
• Resolve filter ability 1 (or 2?)
• Resolve other filter ability

But, mana abilities don't use the stack, and once activated they resolve immediately. So, how can you "respond" to a mana ability in this way?

605.3b An activated mana ability doesn’t go on the stack, so it can’t be targeted, countered, or otherwise responded to. Rather, it resolves immediately after it is activated

Credit to Pucatrade for the situation

Answer 1

It used to be possible to do this by activating the Elf's filter ability during the process of activating the same ability, taking advantage of the fact that one of the steps of casting a spell or activating an ability is rule 601.2g:

If the total cost includes a mana payment, the player then has a chance to activate mana abilities (see rule 605, “Mana Abilities”). Mana abilities must be activated before costs are paid.

But the rule 605.3c introduced in Eldritch Mood explicitly forbids the specific sequence in the question:

Once a player begins to activate a mana ability, that ability can’t be activated again until it has resolved.

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However, it is still possible to do essentially the same thing in a variant scenario where a single creature has two separate instances of the Skyshroud Elf's ability. For example, if you control Ashnod's Altar and Necrotic Ooze, and your graveyard has two copies of Skyshroud Elf in it, then you can accomplish the same thing by doing this:

1. Activate the Ooze's first filter ability. When you get to step 601.2g of the activation process, do the following:
   1. Activate the Ooze's second filter ability. When you get to step 601.2g of this ability's activation process, do the following:
      1. Activate Ashnod's Altar's ability by sacrificing the Ooze. Now you have {C}{C} in your mana pool.
   2. Pay for the second filter ability with {C} and let the ability resolve, adding {W} or {R} to your mana pool. Now you have {C} and {W} or {R} in your mana pool.
2. Pay for the original filter ability with {C} and let the ability resolve to add another {W} or {R} to your mana pool.

By doing this, you end up with two mana in any combination of {W} and {R} to your mana pool.

Answer 2

The relevant rule is

116.1d A player may activate a mana ability whenever he or she has priority, whenever he or she is casting a spell or activating an ability that requires a mana payment, or whenever a rule or effect asks for a mana payment (even in the middle of casting or resolving a spell or activating or resolving an ability).

You announce the first activation of the Elf, then before you pay the costs of that first activation, you announce the second activation. You now have to pay {2}, which you do by sacrificing the elf to the Altar.

Answer 3

No this doesn't work.

602.2. To activate an ability is to put it onto the stack and pay its costs, so that it will eventually resolve and have its effect. Only an object’s controller (or its owner, if it doesn’t have a controller) can activate its activated ability unless the object specifically says otherwise. Activating an ability follows the steps listed below, in order. If, at any point during the activation of an ability, a player is unable to comply with any of those steps, the activation is illegal; the game returns to the moment before that ability started to be activated (see rule 717, “Handling Illegal Actions”). Announcements and payments can’t be altered after they’ve been made.

emphasis added

Mana abilities resolve immediately (605.3b), but you must pay for them first. There is no way to interrupt the activation process; the game rules require one to complete the casting/activation process or rewind the game (602.2; 601.2 for spells)