Xtreme has Per Page Sidebar Width option, but only for single sidebar.
If you want to change the Split Sidebars width on a single page, you need to use the rules below
 
For regular Split Sidebars not using smart margins use

@media screen and (min-width:768px) {
.page-id-xxx .l-sb-right-split{width:20%}
.page-id-xxx .l-content-ssb{width:66%;margin-left:14%;}
.page-id-xxx .l-sb-left-split{margin-left:-80%;width:14%}
}

The way to calculate the values is as follow

• In the first and third rule you set the width of the right and left sidebar (here 20% and 14%)
• In the second rule, the width (of the content) needs to be 100% minus Width of sidebars, here 100-20-14 = 66%
• The margin left is the left sidebar width
• In the third rule, the margin left must be negative of the content width plus content margin-left here 66+14=80%

 
For regular Split sidebar but using Smart margins, use

@media screen and (min-width:768px) {
.page-id-xxx .l-sb-right-split-lm {width:20%}
.page-id-xxx .l-content-ssb{width:63%;margin-left:16%;}
.page-id-xxx .l-sb-left-split-rm {margin-left:-79%;width:15%}
}

The way to calculate the values is as follow

• In the first and third rule you set the width of the right and left sidebar (here 20% and 15%)
• In the second rule, the width (of the content) needs to be 100% minus Width of sidebars and twice smart margin, here 100-20-15-1-1 = 63%
• The margin left is the left sidebar width plus smart margin, here 15+1=16
• In the third rule, the margin left must be negative of the content width plus content margin-left here 63+16=79%

 
For Split Sidebar stacked on top without smart margin use

@media screen and (min-width:768px) {
.page-id-xxx .l-sb-right-split{width:25%}
.page-id-xxx .l-content-ssbs {width:60%;}
.page-id-xxx .l-sb-left-split-top {width:15%}
}

Just make sure the sum of the width is 100%
 
For Split stacked using smart margins use

@media screen and (min-width:768px) {
.page-id-xxx .l-sb-right-split-lm {width:24%}
.page-id-xxx .l-content-ssbs {width:60%;}
.page-id-xxx .l-sb-left-split-top-rm {width:14%}
}

Make sure the total of the width is 100% minus twice the smart margin (1% by default), here 98%