05AB1E, 10 bytes
.ò0¹²g-Ì∍J
.ò is the builtin that already gives the answer for most test cases. 0¹²g-Ì∍J is to correctly append trailing zeroes if necessary.
.ò # Round the second decimal input to the integer first input amount of decimals
# i.e. 10 and 0.1 → 0.1
# i.e. 1 and 0.67231 → 0.7
0 # Literal '0'
¹ # Get the first integer input
²g # Get the length of the second decimal input
- # Subtract them from each other
Ì # Increase it by 2 for the "0." part
# i.e. 10 and 0.1 → 10-3+2 → 9
# i.e. 1 and 0.67231 → 1-7+2 → -8
∍ # Extend the "0" to a size of that length
# i.e. "0" and 9 → "000000000"
# i.e. "0" and -8 → ''
J # Join everything together
# i.e. 0.1 and "000000000" → 0.1000000000
# i.e. 0.7 and '' → 0.7
