Under the new rule, if I cast Rishkar's Expertise, I may cast a split card only if the combined CMC is 5 or smaller, but only one side of it, but if I cast a fuse spell I may cast either or both sides, yes?
The gatherer rulings for the Expertises answer this question:
4/18/2017 The converted mana cost of a split card is determined by the combined mana cost of its two halves. If an expertise spell allows you to cast a split card, you may cast either half or, if that split card has fuse, both halves.
This is a new ruling based on the most recent rules changes. Based on that ruling, the answer to your question is yes.
If a split card with fuse is in your hand, you can still choose to cast both halves of that card rather than cast one of them (C.R. 702.101a). However, Rishkar's Expertise says "cast a card ... from your hand", not "cast a spell ... from your hand", and that "card" refers to the card in your hand (compare C.R. 109.2a with C.R. 109.2b, respectively), sowhat's relevant here is the converted mana cost of the card in your hand (not the converted mana cost of the spell it will become), which will be the total converted mana cost of both halves since "[t]he mana cost of a split card is the combined mana costs of its two halves" (C.R. 708.4b, 202.3)For the purposes of Rishkar's Expertise, what is relevant is what characteristics the corresponding spell has on the stack.
EDIT (May 4): Edited comment since Amonkhet was released.
EDIT (Oct. 1, 2019): See comment 4.
EDIT (May 5, 2020): Edited to conform to update with Ikoria: Lair of Behemoths.
As of Throne of Eldraine, the answer to comment 1 has changed.
When that set is released (and in prereleases), for the purposes of determining whether you can cast a split card from your hand with Rishkar's Expertise, what is relevant are the characteristics of the half you choose (or, if the card has fuse and you so choose, the characteristics of both halves) in your hand before you begin to cast that card (new C.R. 601.3e; C.R. 708.3a, 702.101a). When that set is released (and in prereleases), with Rishkar's Expertise, you can cast from your hand—
any half of a split card, if that half's converted mana cost is 5 or less, or
both halves of a split card, if the card has fuse and their combined converted mana cost is 5 or less.
EDIT (May 5): As of Ikoria: Lair of Behemoths, the answer to comment 1 has changed once again, but not in a manner that seriously affects this answer in general. Now, for the purposes of determining whether you can cast a split card spell from your hand with Rishkar's Expertise, what is relevant are the characteristics of that spell. This will in general be the characteristics of the half you choose (or, if the card has fuse and you so choose, the characteristics of both halves) (C.R. 708.3a, 702.101a).
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This is a new ruling based on the most recent rules changes. Based on that ruling, the answer to your question is yes.
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However, Rishkar's Expertise says "cast a card ... from your hand", not "cast a spell ... from your hand", and that "card" refers to the card in your hand (compare C.R. 109.2a with C.R. 109.2b, respectively), sowhat's relevant here is the converted mana cost of the card in your hand (not the converted mana cost of the spell it will become), which will be the total converted mana cost of both halves since "[t]he mana cost of a split card is the combined mana costs of its two halves" (C.R. 708.4b, 202.3)For the purposes of Rishkar's Expertise, what is relevant is what characteristics the corresponding spell has on the stack.EDIT (May 4): Edited comment since Amonkhet was released.
EDIT (Oct. 1, 2019): See comment 4.
EDIT (May 5, 2020): Edited to conform to update with Ikoria: Lair of Behemoths.
When that set is released (and in prereleases), for the purposes of determining whether you can cast a split card from your hand with Rishkar's Expertise, what is relevant are the characteristics of the half you choose (or, if the card has fuse and you so choose, the characteristics of both halves) in your hand before you begin to cast that card (new C.R. 601.3e; C.R. 708.3a, 702.101a). When that set is released (and in prereleases), with Rishkar's Expertise, you can cast from your hand—
EDIT (May 5): As of Ikoria: Lair of Behemoths, the answer to comment 1 has changed once again, but not in a manner that seriously affects this answer in general. Now, for the purposes of determining whether you can cast a split card spell from your hand with Rishkar's Expertise, what is relevant are the characteristics of that spell. This will in general be the characteristics of the half you choose (or, if the card has fuse and you so choose, the characteristics of both halves) (C.R. 708.3a, 702.101a).