BLAKE SHELTON and MIRANDA LAMBERT are flying high – but friends fear success will be the kiss of death for their marriage!

Shelton, 36, and Lambert, 29, were up-and-coming country singers when they fell in love in 2006, but now sources say their skyrocketing careers – and his wandering eye – threaten to tear them apart.

“Despite being married, Miranda and Blake are like two ships passing in the night,” a friend told The ENQUIRER.

“Between their touring schedules and public appearances, they’re actually together only about 50 days a year. That’s no way to run a marriage – and suspicions and jealousies creep up between them.”

The biggest problem seems to be Blake’s coaching gig on the hit NBC singing competition “The Voice.”

“Blake has become a household name and he spends most of his time in Los Angeles working on ‘The Voice,’ ” said the friend. “But Miranda hates L.A. She prefers to be at their ranch in Oklahoma.”

Besides that, for Blake, coaching on “The Voice” is like giving a fox the keys to the hen house, said the source.

“Blake’s surrounded by dozens of beautiful young singers who’d give anything for him to make them stars,” explained the friend.

A worried Miranda has reportedly warned Blake to stay away from last season’s winner Cassadee Pope, the 23-year-old Florida native that he mentored.

And as The ENQUIRER has recently reported, Miranda was also jealous over her hubby’s flirty relationship with fellow “Voice” coach Shakira.

“Remember, Miranda and Blake began flirting with each other while he was still married,” added the pal. “And there are already lies about his cheating floating around.”

To make matters worse, neither star will give an inch when it comes to making career sacrifices, said the source.

Blake – who’s scored numerous chart-topping hits, including “Honey Bee” – asked Miranda to mentor his team on “The Voice” this season, but she turned him down flat.

“Miranda is touring from now through October,” the source noted. “She said she wasn’t going to give up hundreds of thousands of dollars just to play second banana to Blake on TV.

“But it’s also a case of tit-for-tat. Miranda asked Blake to tour with her as a co-headliner so they’d have more time together, but he refused. He told Miranda, ‘You have your fans, I have mine, and we need to keep our careers separate.’”

GRAMMY WINNER Miranda is taking a five-day break from her tour in mid-May to celebrate the couple’s second wedding anniversary with her hubby. But with Blake heading out on tour after “The Voice” wraps in late June, friends say extended quality time isn’t on the horizon for the couple.

“Blake and Miranda really need to make the effort to give their marriage time to blossom and grow,” said the friend. “As it is now, they’re drifting further and further apart.”