I don't think Aragorn asked about Merry and Pippin. They didn't ask in the book either (I'm asking here). Maybe they didn't ask because they wouldn't expect Gandalf to know.
Yet, no explanation was given for Aragorn to give up his search of Merry and Pippin. Except maybe there was other pressing problems, and Aragorn, relieved that Merry and Pippin were alive believed that they could take of themselves.
"A thing is about to happen which has not happened since the Elder Days: the Ents are going to wake up and find that they are strong.''What will they do?' asked Legolas in astonishment. 'I do not know,' said Gandalf. 'I do not think they know themselves. I wonder.' He fell silent, his head bowed in thought.
The others looked at him. A gleam of sun through fleeting clouds fell on his hands, which lay now upturned on his lap: they seemed to be filled with light as a cup is with water. At last he looked up and gazed straight at the sun. 'The morning is wearing away,' he said. 'Soon we must go.' 'Do we go to find our friends and to see Treebeard?' asked Aragorn.
'No,' said Gandalf. 'That is not the road that you must take."
In a way that works quite well. Lucas, you see, would have bothered to show us every step taken and every word spoken, going to ridiculous lengths to explain everything, rather than leaving room for easy interpolation.
PJ is far more economical and it makes him a btter film maker.
Someone asked in the books. Gandalf explained about the Ents and Legolas got all excited finding out that there were still Ents in the world.