$ cat /Resources/maps_sideScrolling/tileProperties.txt
abc
def
ghi

$ cat /Resources/maps_sideScrolling/ruinsItemCave2.tmx
pre
stuff
<tileset firstgid="1"
foo
bar
<layer name="background"
post
stuff

$ awk '
NR==FNR { s=s ORS $0; next }
/<tileset firstgid="1"/    { print $0 s; f=1 }
/<layer name="background"/ { f=0 }
!f
' /Resources/maps_sideScrolling/tileProperties.txt /Resources/maps_sideScrolling/ruinsItemCave2.tmx
pre
stuff
<tileset firstgid="1"
abc
def
ghi
<layer name="background"
post
stuff