I have lots of CardGroup use in the views like:

        <CardGroup>
            <Card Style="overflow-y:scroll; max-width:500px; max-height:780px;" Border="Border.Is4" Padding="Padding.Is2.OnAll" Height="Height.Is100">
                <CardHeader Background="Background.Light" Style="position:sticky; top:0; opacity:1.0" >
                    <CardTitle>DataItems Selection</CardTitle>
                </CardHeader>
                <CardBody>
                    <DataItemsTree @ref="@dataItemsTreeView" DataItemForm="@dataItemForm" SelectedDataItem="@dataItem_" SelectedVersion="@selectedVersion_"/>
                </CardBody>
            </Card>
            <Card Style="overflow-y:scroll; max-height:780px;" Border="Border.Is4" Padding="Padding.Is2.OnAll" Width="Width.Is100" Height="Height.Is100">
                <CardHeader Background="Background.Light" Style="position:sticky; top:0; opacity:1.0">
                    <CardTitle>DataItem Form</CardTitle>
                </CardHeader>
                <CardBody>
                    <DataItemForm @ref="@dataItemForm"
                        UpdateParent="@OnUpdateParent"
                        ModuleLookup="@moduleLookup_" 
                        FormatLookup="@formatLookup_"
                        DataTypeLookup="@datatypeLookup_" 
                        DataItemClassLookup="@dataItemClassLookup_" 
                        SIUnitLookup="@unitLookup_" 
                        DataItemLookup="@dataItemLookup_" />
                </CardBody>
            </Card>
        </CardGroup>

Is it possible to create a splitter between 2 cards? Thank you.

CardGroup is supposed to have hards stick together, as per the Bootstrap documentation. https://getbootstrap.com/docs/4.6/components/card/#card-groups

If you want more space in between you can use CardDeck instead of CardGroup.

By splitter I mean a way to a way to resize the cards inside a card group. Like this project I found https://github.com/carlfranklin/BlazorSliders.

We don't have splitter component at the moment. I guess you could use BlazorSliders if they are working in a general way and in case they can contain any content.