In this case, the first part should be posted on the right and the second part on the left.
Generally speaking, the tone of the last word is used to distinguish the upper and lower couplets. In standard couplets, the tone of the ending words is up (three or four tones, including the entering tone of ancient rhyme) and down (one or two tones).
When pasting couplets, the position of the upper and lower couplets depends on the format of the horizontal batch. If horizontal batches are traditionally read from right to left, paste the right side. If the horizontal batch is from left to right, stick the couplet on the left.