When do we use two past tense together, as in "consecutive way" ? e.g. had decided?

Answer 1

had decided is past perfect tense. He already had decided to go to the store.

Answer 2

You aren't using two past tenses together in your example.

Most English past tenses are formed using an auxiliary verb plus a participle of some kind.

Such as:
Simple Past
Past Continuous
Present Perfect
Present Perf. Cont.
Past Perfect
Past Perfect Cont.
--
Simple Past [VERB+ed]
You called Bill.
Did you call Bill? (auxiliary =did)
You did not call Bill.

Past Continuous [was/were + present participle]
You were studying when she called.
Were you studying when she called?
You were not studying when she called.

Present Perfect [has/have + past participle]
You have seen that movie many times.
Have you seen that movie many times?
You have not seen that movie many times.

Present Perf. Continuous [has/have + been + present participle]
You have been waiting here for two hours.
Have you been waiting here for two hours?
You have not been waiting here for two hours.

Past Perfect [had + past participle]
You had studied English before you moved to New York.
Had you studied English before you moved to New York?
You had not studied English before you moved to New York.

Past Perfect Continuous [had been + present participle]
You had been waiting there for more than two hours when she finally arrived.
Had you been waiting there for more than two hours when she finally arrived?
You had not been waiting there for more than two hours when she finally arrived.

You might be referring to some FUTURE tense constructions which are more complex. (For example, Future Perfect Continuous has two different forms: "will have been doing " and "be going to have been doing." which is formed by [will have been + present participle]).