That is not possible. You can add both directories to the / partition or you can put /var and /tmp each on a seperate partition.
You can use a symlink and have /tmp store its files on a new directory like /var/temp/. That would require the following commands:
sudo mkdir /var/temp/
sudo rm -rf /tmp/
sudo ln -s /var/temp/ /tmp/
This also requires you to recreate the permissions to /tmp/.
sudo chmod 777 /tmp
sudo chmod +t /tmp/
sudo chown root:root /tmp/
Untested. Use at own risc:possible issue: you might have to set /var/temp/ with the same permissions as /tmp!