The forms used in counting are essentially the N-class forms. The N- before a W becomes mb-. The actual root of "two" is "-wili".
Watu wawili two people
Miti miwili two trees
Maembe mawili two mangoes
Vitu viwili two things
Simba wawili two lions (animate!)
Nyuso mbili two faces
Oh yes, oops, sorry! I'll edit it to not mislead anyone who comes later.
(The N class animate nouns seem to use the N class possessives though, ya, za, yangu etc. and my theory is that it's because the nouns don't show number and the M-WA possessive forms don't either (all starting with w) so it's like a way of maximising the chance that you'll know if it's singular or plural.)