Can I make a buffer from two different weak acids or bases?
Yes, having multiple weak acids and/or bases having different pKa values would give buffering capacity in more than one pH range, or over a greater pK range.  

There is a term "universal buffer" which refers to a buffer having multiple pKas spaced thoughout the pH ~2-12 range to give some buffering capacity over the enter pH range.  

As explained in A Neutral Buffered Standard for Hydrogen Ion Work and Accurate
Titrations Which Can be Prepared in One Minute J. Am. Chem. Soc., 1932, 54, pp 1911–1912, ammonium aceate has the property that if it is added to pure water it produces a buffered solution of pH 7.00, accurate to a hundreth of a pH unit.