spent; reading
句意:他们花了两小时读这本书。
句子表达:做某事花费某人...时间。其对应的句子结构为:It took/takes...time to do sth.
或sb. spend time doing sth.
题干中句子主语为they,故需用动词spend,句子中took为take的过去式,故句子需用一般过去时,需用spend 的过去式,即spent,第二空表达看书需用动词read,此处需用其动名词,故答案为:reading。所以本题答案为:spent...reading.