Count like Chuck Norris
Python 2/C (clang), 109 107 100 84 95 88 89 88 87 84 bytes



i=0;
#/*
while 1:i+=1L;print`i`[::-1]
'''*/
a(){for(;;)printf("%i %1$i ",++i);}//'''


Python: Try it online!

C: Try it online!

The L in the Python code is part of the delimiter.

Explanation:

In the C code, It first sets i to 0. Then, it starts a comment (# is valid code in C for #include statements) where the Python code goes. At the end of the comment, it defines a function that forever increments a variable and prints it twice, space-delimited. It then begins a comment.

In the Python code, i=0; sets i to zero. Python ignores the next line because # starts a single-line comment. It then forever increments it and turns it into a long a number and prints its reversed string representation. The 'L' from the long is part of the delimiter. After that, it starts a multi-line string to comment the C code, that ends later.

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-2 bytes thanks to @LuisMendo. -7 bytes thanks to @ZacharyT. -6 more bytes thanks to @ZacharyT. +11 bytes to fix a bug thanks to @mbomb007. -7 bytes thanks to @Doorknob. +1 byte to fix a bug thanks to @Doorknob. -1 byte thanks to @yoann. -1 more byte thanks to @yoann. -3 bytes thanks to @Cyoce.