Both seem to take the assumption of something before the word, and then take something that the speaker does after the word because the assumption is not met. But I’m not sure if there is any difference between the two. At least I think 텐데 is used more often than 련만.
So what is the difference?
"련만" is an abbreviation of "련마는", and "텐데" is an abbreviation of "터인데", and "터인데" is a combination of "터" and "-인데", which is also decomposed to "이다" and "-ㄴ데".
According to the Standard Korean Language Dictionary, each of them has the following meaning:
련마는 어떤 조건이 충족되면 이러이러한 결과가 기대되는데, 아쉽게도 그 조건이 충족되지 못하여 기대하는 결과도 이루어질 수 없음을 나타내는 연결 어미. (...)
We may use 련마는 when we can expect some result under some conditions, but they are not satisfied with unfortunate reasons, so the result does not happen.
터 ‘예정’이나 ‘추측’, ‘의지’의 뜻을 나타내는 말, ‘처지’나 ‘형편’의 뜻을 나타내는 말.
터 implies expectation, guess, or a will. It also implies a circumstance or position.
ㄴ데 뒤 절에서 어떤 일을 설명하거나 묻거나 시키거나 제안하기 위하여 그 대상과 상관되는 상황을 미리 말할 때에 쓰는 연결 어미.
We may use ㄴ데 to refer to relevant circumstances to ask/order/suggest something.
Hence we can use "텐데" when suggesting a guess or expectation. On the one hand, you may replace "련만" with "텐데". On the other hand, "련만" always blame that has lacked in the past or present, so the expected result does not happen in the present. It means we cannot always replace "텐데" with "련만".
For example, you cannot replace 텐데 in the following sentence
차가 빨리 가야 할 텐데, 그렇지 않으면 지각해.
since it suggests an expectation of the future which can happen as a result of a current situation.
