If you activate the mana ability of Shaun & Rebecca, Agents while in the process of casting a spell, activating an ability, or during a window to activate mana abilities when you're asked to pay a cost during the resolution of a spell or ability, you won't mill cards right away. Instead, a second "reflexive" ability triggers whenever you activate that ability. That ability won't go onto the stack until you're done casting, activating, or resolving that spell or ability, as appropriate. Each player may respond to that triggered ability as normal.