ffmpeg overlay x expression with logical condition
FFmpeg uses functions, rather than operators, for conditional execution and comparisons, so t&lt;2 is not a valid syntax. 

The syntax for your given command would be

ffmpeg -i input.mp4 -i overlay.mov -filter_complex \
       "[0][1]overlay=x=if(lt(t\,2)\,t*3\,t*5)[out]" \
       -map '[out]' -c:v qtrle -y output.mp4


where lt(var,value) is the less than function.