You can grab it with:
var\d+(?:(?!var\d).)*?secondVar
See demo. re.S (or re.DOTALL) modifier must be used with this regex so that . could match a newline. The text between the delimiters will be in Group 1.
NOTE: The closest match will be matched due to (?:(?!var\d).)*? tempered greedy token (i.e. if you have another var + a digit after var + 1+ digits then the match will be between the second var and secondVar.
NOTE2: You might want to use \b word boundaries to match the words beginning with them: \bvar(?:(?!var\d).)*?\bsecondVar.
REGEX EXPLANATION
var - match the starting delimiter
\d+ - 1+ digits
(?:(?!var\d).)*? - a tempered greedy token that matches any char, 0 or more (but as few as possible) repetitions, that does not start a char sequence var and a digit
secondVar - match secondVar literally.
IDEONE DEMO
import re
p = re.compile(r'var\d+(?:(?!var\d).)*?secondVar', re.DOTALL)
test_str = "var12.1\na\na\ndsa\n\n88\n123!!!\nsecondVar12.1\nvar12.1\na\na\ndsa\n\n88\n123!!!\nsecondVar12.1"
print(p.findall(test_str))
Result for the input string (I doubled it for demo purposes):
['12.1\na\na\ndsa\n\n88\n123!!!\n', '12.1\na\na\ndsa\n\n88\n123!!!\n']