Recently, Pole Kubica followed up his Renault test duties with a run in Williams's 2014 car at Silverstone.
Now, it is reported that Kubica, who last raced in F1 in 2010 prior to injuring his arm, has again been testing, this time in Hungary, with Di Resta also present.
Earlier, a Williams spokesman said that the Silverstone test had been "successful", and now he is saying only that the action in Budapest was "productive".
When asked about who will be Lance Stroll's 2018 teammate, Williams technical boss and co-owner Lowe told Brazil's Globo: "I cannot comment on anything right now.
"All I can say - as everyone already knows - is that we have a few options."
Not long ago, Williams had returned as a force in F1, but in 2017 the British team has slumped to fifth overall behind Red Bull and Force India.
Lowe, who started work at Grove this year, said he is focused on taking Williams back to the front of the grid.
When asked if new engineers could be the answer, Lowe told Globo: "We are aware both of what is on the market and the need to strengthen in all areas.
"I have already made some changes in the way the 2018 project was conducted, and at the moment I am satisfied with what I can see."
Lowe, who joined Williams from championship winners Mercedes, said that he is enjoying his role at the once-great team.
"I love my job," he said.
"I've always been employed, but now I have the opportunity to look at the overall project in terms of getting Williams fighting for wins again, not as technical director but also as a partner."
Asked what Williams needs to do to start winning again, he answered: "You need great engineers, an efficient track team, and everything working from an operational point of view.
"When this happens, you get more investors and a better budget."
The 2017 calendar continues with the United States Grand Prix this weekend.