Yes, both Boldtusk and Lancelot add 48% to the attack stat while their special is active.
However, attack buffs don’t stack. So if you used both together, the attack strength of the buffed heroes would only be increased by 48%.
Additionally, if you had another attack buffer, the last attack buff applied is the one that will have effect. That’s true even if it’s a lesser attack.
Wu Kong’s Gambler’s Stance, and Tarlak’s Martial Prowess, do, however, stack with normal attack buffs, since they’re different buffs.