If word order is constrained by the natural order of the sequence of real-world events, there is no word-order flexibility in the serial-verb construction. In other word, the relationship between the two verb phrases is not parallel but is consecutive in nature.
→ 他們上樓睡覺 (t¯a-men sh`ang l´ou shu`ı ji`ao/ They went upstairs and slept.)
Nevertheless, the aspect marker can still be affixed to the first verb or the second verb, making clear the verbal status of the two verb phrases in this serial-verb construction. The placement of a perfective marker -le (了) may imply that the verb with it should then be treated as the main verb of the sentence.
→ 他們上了樓睡覺 (t¯a-men sh`ang-le l´ou shu`ı ji`ao/ They went upstairs to sleep.)
Reference: Chinese: a linguistic introduction by Chao Fen Sun