Using GNU awk for "inplace" editing and ARGIND:

awk -i inplace '
    NR == FNR { map[NR]=$2 }
    NR != FNR { sub(/#P#/,map[ARGIND]) }
1' eg.txt ?.txt 

Using GNU awk for "inplace" editing and ARGIND:

awk -i inplace '
    NR == FNR { map[NR]=$2 }
    NR != FNR { sub(/#P#/,map[ARGIND]) }
1' eg.txt ?.txt

The above assumes the replacement text from eg.txt doesn't contain spaces or &s.