這兩動詞為何可以連在一起？

Face is used as a noun in this sentence. ( ~ not a verb)

2012-09-08 20:15:00 補充：
Examples from Longman Dictionary:

Keith was lying face down on the bed.
face (noun) down = with the face or front towards the ground.

face (verb) down [phrasal verb] = deal in a strong and confident way with someone who opposes you.
The supervisor successfully faced down the mob of angry workers.

2012-09-09 01:40:51 補充：
In American English,
facedown (adverb) = with the face down (facedown is one word)
The body was lying facedown.

Facedown (adverb) is rarely used in British English.

"lie" and "face" are verbs ,but are not connected here:-
The infinitive clause is "to lie with face"
"order" is verb of perception; .if this verb is used in the passive voice,the infinitive takes to,eg:-
The hostages were ordered to lie with face down on the ground.
Hence:-The robber ordered the hostages to lie face down on the ground.

你get錯左喇
呢到, face 唔係verb
呢到亦唔係"face down"
係facedown
facedown 係adj = 面向下地
face down係phrase: 承受挫敗；制服，制止
呢到黎講, facedown 係岩
The robber ordered the hostages to lie face down on the ground. 之中的lie face down係唔成立, 冇得解:)