A man has two sons, and another man has two daughters. Is it permissible for the first man to marry his sons to the second man's daughters?
Is this considered combining two sisters in marriage at the same time, prohibited in Allah's statement,
...and two sisters in wedlock at the same time
If the situation is as you mentioned, that a man has two sons and another man has two daughters, it is permissible for each of the sons to marry one of the daughters, if they have not shared suckling from the same woman. This does not entail combining two sisters in marriage at the same time, as each one of them is going to marry a different man. What is prohibited is both sisters marrying the same man at the same time. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.