Responding to 1NT with 5-5 in the majors
What is standard?
Standard treatment of responses with a 5-5 major-suited hand varies. The most common approach is to use 3♥ and 3♠ bids to show a 5-5 hand with invitational and game-forcing values.
This assumes that you do not have a conventional meaning for 3♥ and 3♠ responses. In Modern American, we use those bids for a Puppet Splinter (showing shortness in the major-suit bid, promising 3 cards in the other suit, and game values).
The next common treatment is to transfer partner to spades, then bid 4♥ to show you have game, and partner can pass or correct. This has a couple of weaknesses. First, if partner accepts hearts as the best contract, you will declare and put the strong no-trump hand down as dummy. The second flaw is that partner can pass 4♥; if you were thinking about slam exploration, it just died.
Bidding just 3♥ after the transfer to spades shows a 5-4 hand, and then you'd have to rebid 4♥ after partner's response (assuming it wasn't 4♥). You have still stolen the heart contract and still have issues exploring for a major suit slam
Finally, you can transfer to hearts and then bid 2♠. Discuss with your partner whether this is a game-forcing bid, an invitational bid (showing 5 hearts and 4+ spades), and how they should respond. If they bid 3♥, how will they interpret a 3♠ rebid (to pattern out 5-5, but what is the HCP?)
As you can see, 'standard' is misleading. These hands are difficult to bid, and that doesn't account for the difficulty of determining a true valuation of the hand. How valuable is the shape? Many hands with just 7 or 8 HCP will make game with an 8-card fit and well-placed shortness, or a double fit in both majors.
Defining jump 3♦ over 1NT for majors
What is the solution? We can use the 3♦ bid artificially over 1NT to show a 5-5 major-suit hand. Partner will bid whichever major suit they prefer and therefore will declare the contract (game or slam). This artificial response generally promises game values and may have game+ for slam interest. This improves our auctions with hands below game value. With 'weak' hands, we can bid 3♦, intending to pass our partner at the 3-level. This avoids any possible 5-2 trump fit. Therefore, with invitational hands, we can transfer to hearts and then bid 2♠. Partner will bid again (2NT or 3H with a minimal NT opening hand). After a 2NT bid, the opener shows a minimal hand and denies 3 hearts and denies 4 spades. A response of 3♠ over the 2NT should be a 'to play' bid. A 5-3 fit is implied. After opener's 3♥ rebid, the responder can safely pass, knowing a fit is found. There is no reason to pattern out to look for a 5-3 spade fit.
They can bid 3NT or 4♥ with a full 17 HCP. If they bid 3NT, they are denying 3 hearts, and since we never open 1NT with a doubleton in both majors, they must have 3 spades. We can safely correct the 3NT to 4♥.
We don't lose the ability to bid 5-5 minor-suit hands (a usual definition of the 3♦ jump bid over 1NT is to show 5-5 minor-suit hands with game/slam interest). We have our Smolen with Minors treatment available to pattern out two-suited minor hands.
Given the many ways the field may respond to a 5-5 major-suit hand, it's hard to predict the average (or above-average) solution. When you can't 'read the room' to predict how others will bid with your hand, fall back on our scientific valuation principles. Consider your LTC and HCP. The 3♦ response lets you ask the opener questions; they have already provided a detailed picture of their hand with the 1NT opening bid. 3♦ allows them to show their best major. You retain the captaincy to move the auction along. With a weak hand, you plan to pass partner's 3-level response. You may win the day by finding the better major to play (even if you're at 3 spades and the field is at 2 hearts). Playing 5-3 fit vs. a 5-2 fit could swing you a top board. When the board is cold in a 4-level game, most techniques will lead to it; however, you might get a good result (overtrick) by not revealing your partner's strong hand as dummy and making the opening lead come up to the declarer. Finally, with a strong responder hand (slam interest), you have more descriptive rebids available after partner tells you where the major suit fit is. In just 3 bids (1NT-3♦-3M), you know which major; partner knows you have 10 cards in the majors (therefore 0-3 or 2-1 in the minors). When you don't pass your partner, they also know you are captain, and your next bid signals whether you want to explore slam or settle into the game contract.