 You need to hold ⇧ and press the other key for the 2nd function. It’s easier to tap the same key twice for the 2nd function.

Magic Number  ▸  Preferences… ▸  Shortcuts In general, any function that requires pressing the shift (⇧) key can be accessed via Double Key.

Trigonometry

 S=sin SS=sin⁻¹ C=cos CC=cos⁻¹ T=tan TT=tan⁻¹

Exponents

 Y=xY YY=x1/Y Z=x2 ZZ=x3 J=√ JJ=n√ E=ex EE=2x EEE=10x L=log LL=log₂ LLL=log₁₀

Tax*

 V=tax VV=tax2 VVV=tax3

* Shortcut and names are customizable. See here.

Memory

 A=a AA=a2 B=b BB=b2 K=k KK=k2 M=m MM=m2 N=n NN=n2

The buttons    and can be customized with extra functions (see here).

Their keyboards shortcuts are F1 and F2.
You may find it easier to use F and G instead.

If you right-click or control click   , you get a menu of functions with the related ones grouped together: You can use Double Key to access related functions.
For example:

If: Then:
F1=Modulo F1F1=Quotient
F1=Percent Change F1F1=As Percentage Of
F1=Combinations F1F1=Permutations
F1=Round F1F1=Truncate

It works the other way too. If  F1 = Quotient, then F1 F1 = Modulo. The same applies to F2.

Note:  The keys F1 and F2 are often used by OS X.
We recommend you use F and G.