ue4.24.3

もっといい方法があるかもだけど
左右にそれぞれ複数ウィジェットを揃えたい場合
こんな感じでいけた

sample

SNew(SVerticalBox)
+ SVerticalBox::Slot().AutoHeight()
[
	SNew(SHorizontalBox)
	+ SHorizontalBox::Slot().HAlign(HAlign_Left)
	[
		// 左揃え
		SNew(SHorizontalBox)
		+ SHorizontalBox::Slot()
		[
			SNew(SButton).Text(LOCTEXT("LeftButton1", "LeftButton1")).OnClicked_Raw(this, &FSample::OnButtonClicked)
		]
		+ SHorizontalBox::Slot()
		[
			SNew(SButton).Text(LOCTEXT("LeftButton2", "LeftButton2")).OnClicked_Raw(this, &FSample::OnButtonClicked)
		]
		+ SHorizontalBox::Slot()
		[
			SNew(SButton).Text(LOCTEXT("LeftButton3", "LeftButton3")).OnClicked_Raw(this, &FSample::OnButtonClicked)
		]
	]

	+ SHorizontalBox::Slot().HAlign(HAlign_Right)
	[
		// 右揃え
		SNew(SHorizontalBox)
		+ SHorizontalBox::Slot()
		[
			SNew(SButton).Text(LOCTEXT("RightButton1", "RightButton1")).OnClicked_Raw(this, &FSample::OnButtonClicked)
		]
		+ SHorizontalBox::Slot()
		[
			SNew(SButton).Text(LOCTEXT("RightButton2", "RightButton2")).OnClicked_Raw(this, &FSample::OnButtonClicked)
		]
	]
]